Set rails_env if parts of capistrano-rails are required
Previously, `rails_env` was set only on `capistrano/rails` required.
That caused issues when developers required only `capistrano/rails/assets` or migrations. Fixed #699
Merge pull request #21 from kirs/set-rails-env-fix

Set rails_env if parts of capistrano-rails are required
Fixed cleanup_assets task
Merge pull request #24 from CodingFu/patch-2

Fixed cleanup_assets task