Simplify code that loads a cookbook version

We simply rely on a Chef library now. Deleting code feels so good!

Thanks @kamaradclimber for the original pull request: #26
Merge branch 'master' into get_local_cookbook_version_from_chef
Merge pull request #28 from bmarini/get_local_cookbook_version_from_chef

Get local cookbook version from Chef

It makes the code much simpler and finally fixes #5 (support loading `metadata.json` when no `metadata.rb` file exists)!