Update rails-tutorial.md

- Include callback_path option when creating the oauth provider since that creates silent failure unless the app uses the gem default route
- Don't generate the failure action since it's never used for anything (and also is not in the Sinatra app etc)
- Use Auth0Controller#callback rather than CallbackController#store for obvious reasons
- Make sure the routes recommended actually match the auth0 application default
- No need for request.env as env is readily available
- redirect_to root_path since we can not assume there to be a users route
- Use Gemfile instead of RubyGems for installing gem