Use activemodel 4.0

Gemfile.lock

@@ -2,29 +2,37 @@ PATH
   remote: .
   specs:
     entasis (1.0.1)
-      activemodel (~> 3.1)
+      activemodel (~> 4.0)
 
 GEM
   remote: https://rubygems.org/
   specs:
-    activemodel (3.2.13)
-      activesupport (= 3.2.13)
-      builder (~> 3.0.0)
-    activesupport (3.2.13)
-      i18n (= 0.6.1)
-      multi_json (~> 1.0)
-    builder (3.0.4)
-    diff-lcs (1.2.1)
-    i18n (0.6.1)
-    multi_json (1.7.2)
-    rspec (2.13.0)
-      rspec-core (~> 2.13.0)
-      rspec-expectations (~> 2.13.0)
-      rspec-mocks (~> 2.13.0)
-    rspec-core (2.13.0)
-    rspec-expectations (2.13.0)
+    activemodel (4.0.0)
+      activesupport (= 4.0.0)
+      builder (~> 3.1.0)
+    activesupport (4.0.0)
+      i18n (~> 0.6, >= 0.6.4)
+      minitest (~> 4.2)
+      multi_json (~> 1.3)
+      thread_safe (~> 0.1)
+      tzinfo (~> 0.3.37)
+    atomic (1.1.13)
+    builder (3.1.4)
+    diff-lcs (1.2.4)
+    i18n (0.6.5)
+    minitest (4.7.5)
+    multi_json (1.7.9)
+    rspec (2.14.1)
+      rspec-core (~> 2.14.0)
+      rspec-expectations (~> 2.14.0)
+      rspec-mocks (~> 2.14.0)
+    rspec-core (2.14.5)
+    rspec-expectations (2.14.2)
       diff-lcs (>= 1.1.3, < 2.0)
-    rspec-mocks (2.13.0)
+    rspec-mocks (2.14.3)
+    thread_safe (0.1.2)
+      atomic
+    tzinfo (0.3.37)
 
 PLATFORMS
   ruby

entasis.gemspec

@@ -16,6 +16,6 @@ Gem::Specification.new do |s|
   s.files                 = Dir.glob("lib/**/*") + %w(Gemfile entasis.gemspec README.md HISTORY LICENSE)
   s.test_files            = Dir.glob('spec/*')
 
-  s.add_runtime_dependency 'activemodel', '~> 3.1'
+  s.add_runtime_dependency 'activemodel', '~> 4.0'
   s.add_development_dependency 'rspec', '~> 2.0'
 end