Drop support to Ruby 1.9.3

Author: maurogeorge

CHANGELOG.md

@@ -2,6 +2,7 @@
 
 Breaking changes:
 
+- Drop support to Ruby 1.9.3 (@maurogeorge)
 - [#1131](https://github.com/rails-api/active_model_serializers/pull/1131) Remove Serializer#root_name (@beauby)
 - [#1138](https://github.com/rails-api/active_model_serializers/pull/1138) Introduce Adapter::Base (@bf4)
   * Adapters now inherit Adapter::Base. 'Adapter' is now a module, no longer a class.

active_model_serializers.gemspec

@@ -18,7 +18,7 @@ Gem::Specification.new do |spec|
   spec.test_files    = spec.files.grep(%r{^(test|spec|features)/})
   spec.require_paths = ['lib']
 
-  spec.required_ruby_version = '>= 1.9.3'
+  spec.required_ruby_version = '>= 2.0.0'
 
   rails_versions = '>= 4.0'
   spec.add_runtime_dependency 'activemodel', rails_versions