Upgrade path for railties prefix change

This was super frustrating to figure out. If you don't fix the "rails/" namespace, rubygems tries to activate an old system gem and dies with the worst error message.

$ rake --trace        
(in /Volumes/37signals/foo)
rake aborted!
undefined method `map' for nil:NilClass
/Users/josh/.rvm/ruby-1.8.7-p174/lib/ruby/site_ruby/1.8/rubygems.rb:270:in `activate'
/Users/josh/.rvm/ruby-1.8.7-p174/lib/ruby/site_ruby/1.8/rubygems/custom_require.rb:35:in `require'
/Volumes/37signals/foo/Rakefile:10

The error is caused by our rubygem hackery. We probably want to hack loaded_stacks too to fix this message.

Gem.instance_variable_get("@loaded_stacks")[stub] = []