Force symbols from deploy.rb into strings to be YAML compatible

From:
https://github.com/timkelty/capistrano-db-tasks/pull/1

        It's idiomatic Ruby to use symbols when possible. rails_env is usually a symbol in deploy.rb, but using a symbol makes it incompatible with the remote database.yml.

        I've stringified @cap.rails_env to fix that issue, and symbols can be used again.