Use RAILS_VERSION on Gemfile

Gemfile

@@ -17,4 +17,11 @@ platforms :jruby do
   gem 'activerecord-jdbcsqlite3-adapter'
 end
 
-gem 'rails', '~> 4.0.0'
+version = ENV["RAILS_VERSION"] || "4.0.2"
+rails = case version
+        when "master"
+          {:github => "rails/rails"}
+        else
+          "~> #{version}"
+        end
+gem "rails", rails