Replace railsbridge dead link with new one

Railsbridge [Create and Deploy a Rails App (wayback)](http://web.archive.org/web/20130622140324/http://installfest.railsbridge.org/installfest/create_and_deploy_a_rails_app)  article linked in that section is no longer in use (and broken), having been replaced by [Deploying to heroku](http://docs.railsbridge.org/intro-to-rails/deploying_to_heroku).