Support loading metadata.rb in opscode worker

We have an internal app that serves up a Opscode community API-compatible cache of our internal cookbooks.  New cookbook versions are staged there and are available for testing before they are uploaded to our Chef server.

I'd like to move to Berkshelf 3 and the API, but our cached cookbooks have metadata.rb files, not compiled metadata.json. This pull request adds fallback support for reading metadata from metadata.rb if metadata.json doesn't exist.  The loading logic is pulled from the github cache worker.