Update phpunit/phpunit requirement to 6.3.*|6.5.* (#1)

Updates the requirements on [phpunit/phpunit](https://github.com/sebastianbergmann/phpunit) to permit the latest version.
- [Changelog](https://github.com/sebastianbergmann/phpunit/blob/master/ChangeLog-5.7.md)
- [Commits](https://github.com/sebastianbergmann/phpunit/commits/6.5.5)

composer.json

@@ -16,7 +16,7 @@
         "willdurand/geocoder": "^4.0"
     },
     "require-dev": {
-        "phpunit/phpunit": "6.3.*",
+        "phpunit/phpunit": "6.3.*|6.5.*",
         "geocoder-php/provider-integration-tests": "^1.0",
         "php-http/message": "^1.0",
         "php-http/curl-client": "^1.7",