Fix the Rails environment dependency resolve

We were sharing the cache per enviroment when we wanted to make the
rails env part of the cache.

Fixes #325.

Author: rafaelfranca

lib/sprockets/railtie.rb

@@ -113,10 +113,11 @@ def configure(&block)
     end
 
     Sprockets.register_dependency_resolver 'rails-env' do
-      ::Rails.env
+      ::Rails.env.to_s
     end
+
     config.assets.configure do |env|
-      env.depend_on 'environment-version'
+      env.depend_on 'rails-env'
     end
 
     config.assets.configure do |env|