Set rails_env if parts of capistrano-rails are required

Previously, `rails_env` was set only when `capistrano/rails` was required.
That caused issues when developers required only `capistrano/rails/assets` or migrations. Fixes https://github.com/capistrano/capistrano/pull/699 and https://github.com/capistrano/capistrano/issues/724

/cc @leehambley