Suggest users to require 'capistrano/rails'

Instead of `require 'capistrano/rails/assets'` and `require 'capistrano/rails/migrations'` to fix bug when `rails_env` was not set in [capistrano/rails/lib/capistrano/tasks/rails.rake](https://github.com/capistrano/rails/blob/master/lib/capistrano/tasks/rails.rake#L2)