Use master as default activemodel version

Author: mrcasals

Gemfile

@@ -3,10 +3,10 @@ source "https://rubygems.org"
 gemspec
 
 active_model_opts =
-  case ENV['ACTIVE_MODEL_VERSION']
+  case version = ENV['ACTIVE_MODEL_VERSION'] || "master"
   when 'master' then { github: 'rails/rails' }
   when 'default' then '~> 3'
-  else "~> #{ENV['ACTIVE_MODEL_VERSION']}"
+  else "~> #{version}"
   end
 
 gem 'activemodel', active_model_opts