diff --git a/.ruby-version b/.ruby-version
index 5859406..197c4d5 100644
--- a/.ruby-version
+++ b/.ruby-version
@@ -1 +1 @@
-2.2.3
+2.4.0
diff --git a/Gemfile.lock b/Gemfile.lock
index d3e4ec7..4288092 100644
--- a/Gemfile.lock
+++ b/Gemfile.lock
@@ -4,7 +4,7 @@ GEM
     mini_portile2 (2.1.0)
     nokogiri (1.7.0.1)
       mini_portile2 (~> 2.1.0)
-    power_assert (0.4.1)
+    power_assert (1.0.1)
     rake (12.0.0)
     test-unit (3.2.3)
       power_assert
@@ -18,4 +18,4 @@ DEPENDENCIES
   test-unit
 
 BUNDLED WITH
-   1.13.6
+   1.14.3
diff --git a/circle.yml b/circle.yml
index 9b3148a..30cc426 100644
--- a/circle.yml
+++ b/circle.yml
@@ -1,3 +1,15 @@
+machine:
+  environment:
+    LD_LIBRARY_PATH: /usr/local/lib
+    RAMS_TEST_CBC: true
+    RAMS_TEST_CLP: true
+    RAMS_TEST_GLPK: true
+    RAMS_TEST_SCIP: true
+  
+  ruby:
+    version:
+      2.4.0
+
 dependencies:
   cache_directories:
     - glpk-4.60
@@ -16,4 +28,4 @@ dependencies:
 
 test:
   post:
-    - LD_LIBRARY_PATH=/usr/local/lib bundle exec rake test
+    - bundle exec rake test
diff --git a/lib/rams/numeric.rb b/lib/rams/numeric.rb
index c3da023..0d4d2d9 100644
--- a/lib/rams/numeric.rb
+++ b/lib/rams/numeric.rb
@@ -1,9 +1,9 @@
-# Fixnums can be added to expressions or variables or multiplied
+# Integer can be added to expressions or variables or multiplied
 # by them to create expressions:
 #
 #     4 - (3 * x)
 #     y / 2
-class Fixnum
+class Integer
   alias old_add +
   alias old_sub -
   alias old_multiply *
diff --git a/rams.gemspec b/rams.gemspec
index 3bf87dd..14691f0 100644
--- a/rams.gemspec
+++ b/rams.gemspec
@@ -4,7 +4,7 @@ $LOAD_PATH.unshift(lib) unless $LOAD_PATH.include?(lib)
 
 Gem::Specification.new do |spec|
   spec.name          = 'rams'
-  spec.version       = '0.1.5'
+  spec.version       = '0.1.6'
   spec.authors       = ["Ryan J. O'Neil"]
   spec.email         = ['ryanjoneil@gmail.com']
   spec.summary       = 'Ruby Algebraic Modeling System'
@@ -14,4 +14,6 @@ Gem::Specification.new do |spec|
 
   spec.files         = Dir['lib/**/*']
   spec.require_paths = ['lib']
+
+  spec.required_ruby_version = '>= 2.4.0'
 end
diff --git a/tests/test_model.rb b/tests/test_model.rb
index 3ac4d05..68afc78 100644
--- a/tests/test_model.rb
+++ b/tests/test_model.rb
@@ -5,48 +5,48 @@
 # rubocop:disable ClassLength
 class TestModel < Test::Unit::TestCase
   def test_simple
-    run_test_simple :cbc
-    run_test_simple :clp
+    run_test_simple :cbc if ENV['RAMS_TEST_CBC']
+    run_test_simple :clp if ENV['RAMS_TEST_CLP']
     run_test_simple :cplex if ENV['RAMS_TEST_CPLEX']
-    run_test_simple :glpk
-    run_test_simple(:scip, ['-c', 'set presolving maxrounds 0'])
+    run_test_simple :glpk if ENV['RAMS_TEST_GLPK']
+    run_test_simple(:scip, ['-c', 'set presolving maxrounds 0']) if ENV['RAMS_TEST_SCIP']
   end
 
   def test_binary
-    run_test_binary :cbc
+    run_test_binary :cbc if ENV['RAMS_TEST_CBC']
     run_test_binary :cplex if ENV['RAMS_TEST_CPLEX']
-    run_test_binary :glpk
-    run_test_binary :scip
+    run_test_binary :glpk if ENV['RAMS_TEST_GLPK']
+    run_test_binary :scip if ENV['RAMS_TEST_SCIP']
   end
 
   def test_integer
-    run_test_integer :cbc
+    run_test_integer :cbc if ENV['RAMS_TEST_CBC']
     run_test_integer :cplex if ENV['RAMS_TEST_CPLEX']
-    run_test_integer :glpk
-    run_test_integer :scip
+    run_test_integer :glpk if ENV['RAMS_TEST_GLPK']
+    run_test_integer :scip if ENV['RAMS_TEST_SCIP']
   end
 
   def test_infeasible
-    run_test_infeasible :cbc
-    run_test_infeasible :clp
+    run_test_infeasible :cbc if ENV['RAMS_TEST_CBC']
+    run_test_infeasible :clp if ENV['RAMS_TEST_CLP']
     run_test_infeasible :cplex if ENV['RAMS_TEST_CPLEX']
-    run_test_infeasible :glpk
-    run_test_infeasible :scip
+    run_test_infeasible :glpk if ENV['RAMS_TEST_GLPK']
+    run_test_infeasible :scip if ENV['RAMS_TEST_SCIP']
   end
 
   def test_unbounded
-    run_test_unbounded :cbc
-    run_test_unbounded :clp
+    run_test_unbounded :cbc if ENV['RAMS_TEST_CBC']
+    run_test_unbounded :clp if ENV['RAMS_TEST_CLP']
     run_test_unbounded :cplex if ENV['RAMS_TEST_CPLEX']
-    run_test_unbounded :glpk
-    run_test_unbounded :scip
+    run_test_unbounded :glpk if ENV['RAMS_TEST_GLPK']
+    run_test_unbounded :scip if ENV['RAMS_TEST_SCIP']
   end
 
   def test_implication
-    run_test_implication :cbc
+    run_test_implication :cbc if ENV['RAMS_TEST_CBC']
     run_test_implication :cplex if ENV['RAMS_TEST_CPLEX']
-    run_test_implication :glpk
-    run_test_implication :scip
+    run_test_implication :glpk if ENV['RAMS_TEST_GLPK']
+    run_test_implication :scip if ENV['RAMS_TEST_SCIP']
   end
 
   # rubocop:disable MethodLength