remove the thread_safe dependency. Relay on rails for this

Author: Lachlan Sylvester

active_model_serializers.gemspec

@@ -19,7 +19,7 @@ Gem::Specification.new do |spec|
   spec.require_paths = ["lib"]
 
   spec.add_dependency "activemodel", ">= 4.0"
-  spec.add_dependency 'thread_safe','~> 0.3', '>= 0.3.4'
+
   spec.add_development_dependency "rails", ">= 4.0"
   spec.add_development_dependency "bundler", "~> 1.6"
   spec.add_development_dependency "rake"