Finally I found (luckly), I think it's a rails's problem on rc1, to solve it put this in your Gemfile
gem 'rails', github: 'rails/rails'
Then you will get another error
NameError: uninitialized constant ActionDispatch::Session::EncryptedCookieStore
You can solve this changing your 'initializers/session_store.rb' to
Foo::Application.config.session_store :cookie_store, key: '_foo_session'
(pay attention at your app's name in the correct place, don't use 'Foo')
Enjoy!