Add Chef as dependency

Chef was previously included by the Berkshelf gem, however they have recently removed it as a dependency:
RiotGames/berkshelf@be297279ed29b775659a7aa5aef3f9becb1fa68f

This means that Strainer won't work with the latest version of Berkshelf, so I have added the Chef gem as a runtime dependency to fix that.
I put `~> 11.4` as the version constraint, but I'm not sure how you feel about that. Technically it should work just as well with `> 10`, but I thought that would be too relaxed as a version constraint.