diff --git a/.travis.yml b/.travis.yml
index 0466c6a6..c4465ad5 100644
--- a/.travis.yml
+++ b/.travis.yml
@@ -8,7 +8,7 @@ php:
   - 7.4
 env:
   global:
-    - ES_VERSION=6.7.1 ES_DOWNLOAD_URL=https://artifacts.elastic.co/downloads/elasticsearch/elasticsearch-${ES_VERSION}.tar.gz
+    - ES_VERSION=6.8.2 ES_DOWNLOAD_URL=https://artifacts.elastic.co/downloads/elasticsearch/elasticsearch-${ES_VERSION}.tar.gz
 install:
   - wget ${ES_DOWNLOAD_URL}
   - tar -xzf elasticsearch-${ES_VERSION}.tar.gz
diff --git a/composer.json b/composer.json
index 403c686d..1ebacdc8 100644
--- a/composer.json
+++ b/composer.json
@@ -12,11 +12,11 @@
     ],
     "require": {
         "php": "^7.0",
-        "symfony/serializer": "^3.0|^4.0|^5.0",
-        "paragonie/random_compat": "*"
+        "symfony/serializer": "^3.0|^4.0",
+        "paragonie/random_compat": "*",
+        "elasticsearch/elasticsearch": "^6.0"
     },
     "require-dev": {
-        "elasticsearch/elasticsearch": "^6.0",
         "phpunit/phpunit": "^7.0",
         "squizlabs/php_codesniffer": "^3.0"
     },
diff --git a/src/SearchEndpoint/AggregationsEndpoint.php b/src/SearchEndpoint/AggregationsEndpoint.php
index 3e035f43..079a6a5d 100644
--- a/src/SearchEndpoint/AggregationsEndpoint.php
+++ b/src/SearchEndpoint/AggregationsEndpoint.php
@@ -27,7 +27,7 @@ class AggregationsEndpoint extends AbstractSearchEndpoint
     /**
      * {@inheritdoc}
      */
-    public function normalize(NormalizerInterface $normalizer, string $format = null, array $context = [])
+    public function normalize(NormalizerInterface $normalizer, $format = null, array $context = [])
     {
         $output = [];
         if (count($this->getAll()) > 0) {
diff --git a/src/SearchEndpoint/HighlightEndpoint.php b/src/SearchEndpoint/HighlightEndpoint.php
index aa57b3d7..f2b8d8e9 100644
--- a/src/SearchEndpoint/HighlightEndpoint.php
+++ b/src/SearchEndpoint/HighlightEndpoint.php
@@ -37,7 +37,7 @@ class HighlightEndpoint extends AbstractSearchEndpoint
     /**
      * {@inheritdoc}
      */
-    public function normalize(NormalizerInterface $normalizer, string $format = null, array $context = [])
+    public function normalize(NormalizerInterface $normalizer, $format = null, array $context = [])
     {
         if ($this->highlight) {
             return $this->highlight->toArray();
diff --git a/src/SearchEndpoint/InnerHitsEndpoint.php b/src/SearchEndpoint/InnerHitsEndpoint.php
index 8428bd88..f1562fe6 100644
--- a/src/SearchEndpoint/InnerHitsEndpoint.php
+++ b/src/SearchEndpoint/InnerHitsEndpoint.php
@@ -27,7 +27,7 @@ class InnerHitsEndpoint extends AbstractSearchEndpoint
     /**
      * {@inheritdoc}
      */
-    public function normalize(NormalizerInterface $normalizer, string $format = null, array $context = [])
+    public function normalize(NormalizerInterface $normalizer, $format = null, array $context = [])
     {
         $output = [];
         if (count($this->getAll()) > 0) {
diff --git a/src/SearchEndpoint/PostFilterEndpoint.php b/src/SearchEndpoint/PostFilterEndpoint.php
index de3e263c..6055b7ed 100644
--- a/src/SearchEndpoint/PostFilterEndpoint.php
+++ b/src/SearchEndpoint/PostFilterEndpoint.php
@@ -26,7 +26,7 @@ class PostFilterEndpoint extends QueryEndpoint
     /**
      * {@inheritdoc}
      */
-    public function normalize(NormalizerInterface $normalizer, string $format = null, array $context = [])
+    public function normalize(NormalizerInterface $normalizer, $format = null, array $context = [])
     {
         if (!$this->getBool()) {
             return null;
diff --git a/src/SearchEndpoint/QueryEndpoint.php b/src/SearchEndpoint/QueryEndpoint.php
index b7626910..6212b05d 100644
--- a/src/SearchEndpoint/QueryEndpoint.php
+++ b/src/SearchEndpoint/QueryEndpoint.php
@@ -39,7 +39,7 @@ class QueryEndpoint extends AbstractSearchEndpoint implements OrderedNormalizerI
     /**
      * {@inheritdoc}
      */
-    public function normalize(NormalizerInterface $normalizer, string $format = null, array $context = [])
+    public function normalize(NormalizerInterface $normalizer, $format = null, array $context = [])
     {
         if (!$this->filtersSet && $this->hasReference('filter_query')) {
             /** @var BuilderInterface $filter */
diff --git a/src/SearchEndpoint/SortEndpoint.php b/src/SearchEndpoint/SortEndpoint.php
index cd00bb2f..2461c858 100644
--- a/src/SearchEndpoint/SortEndpoint.php
+++ b/src/SearchEndpoint/SortEndpoint.php
@@ -26,7 +26,7 @@ class SortEndpoint extends AbstractSearchEndpoint
     /**
      * {@inheritdoc}
      */
-    public function normalize(NormalizerInterface $normalizer, string $format = null, array $context = [])
+    public function normalize(NormalizerInterface $normalizer, $format = null, array $context = [])
     {
         $output = [];
 
diff --git a/src/SearchEndpoint/SuggestEndpoint.php b/src/SearchEndpoint/SuggestEndpoint.php
index 6d292239..2e2e16ac 100644
--- a/src/SearchEndpoint/SuggestEndpoint.php
+++ b/src/SearchEndpoint/SuggestEndpoint.php
@@ -27,7 +27,7 @@ class SuggestEndpoint extends AbstractSearchEndpoint
     /**
      * {@inheritdoc}
      */
-    public function normalize(NormalizerInterface $normalizer, string $format = null, array $context = [])
+    public function normalize(NormalizerInterface $normalizer, $format = null, array $context = [])
     {
         $output = [];
         if (count($this->getAll()) > 0) {
diff --git a/src/Serializer/Normalizer/CustomReferencedNormalizer.php b/src/Serializer/Normalizer/CustomReferencedNormalizer.php
index 99ca63db..6cd24a8e 100644
--- a/src/Serializer/Normalizer/CustomReferencedNormalizer.php
+++ b/src/Serializer/Normalizer/CustomReferencedNormalizer.php
@@ -26,7 +26,7 @@ class CustomReferencedNormalizer extends CustomNormalizer
     /**
      * {@inheritdoc}
      */
-    public function normalize($object, string $format = null, array $context = [])
+    public function normalize($object, $format = null, array $context = [])
     {
         $object->setReferences($this->references);
         $data = parent::normalize($object, $format, $context);
@@ -38,7 +38,7 @@ public function normalize($object, string $format = null, array $context = [])
     /**
      * {@inheritdoc}
      */
-    public function supportsNormalization($data, string $format = null)
+    public function supportsNormalization($data, $format = null)
     {
         return $data instanceof AbstractNormalizable;
     }
diff --git a/src/Serializer/OrderedSerializer.php b/src/Serializer/OrderedSerializer.php
index 7bc51118..bcbb9c3d 100644
--- a/src/Serializer/OrderedSerializer.php
+++ b/src/Serializer/OrderedSerializer.php
@@ -22,7 +22,7 @@ class OrderedSerializer extends Serializer
     /**
      * {@inheritdoc}
      */
-    public function normalize($data, string $format = null, array $context = [])
+    public function normalize($data, $format = null, array $context = [])
     {
         return parent::normalize(
             is_array($data) ? $this->order($data) : $data,