Use Ruby/YAML to parse the Procfile

I have comments in my Procfile and they brake the bash string manipulation techniques used in the post-release hook ([relevant lines](https://github.com/statianzo/dokku-supervisord/blob/master/post-release#L19-L20)).

I know it's tempting to just "fix" them to take comments into account, but my advice is to just use a real YAML parser (as [is done in buildstep itself](https://github.com/progrium/buildstep/blob/master/stack/builder#L60-L92)).

Probably the best option is to just use Ruby which is already bundled inside the builder image (I can help if needed). :)