Bump Gemfile to use Ironfan 4.12 & Chef 11 & Berks 2.0

Bumped Ironfan gem version to ~> 4.12.2
Bumped Chef gem version to ~> 11.6.0
Bumped berkshelf version to ~> 2.0.10

You need to also get the updated version of ironfan that has relaxed the gem requirements to allow for Chef 11 that I sent pull request https://github.com/infochimps-labs/ironfan/pull/312

Also right now json has to be pinned to 1.7.7 to eliminate some crazy insane conflict between Berkshelf and Chef 11 even though their constraints don't actually require it. See https://github.com/RiotGames/berkshelf/issues/676