[berkshelf] bump version to 2.0.12

`berkshelf` 1.4.3 was VERY old and began pulling in an incompatible 
`faraday` 0.9.0. The 2.0.x series of `berkshelf` should remain 
compatible with existing 1.4.x Berksfiles.
Merge pull request #116 from opscode/schisamo/berkshelf-update

[berkshelf] bump version to 2.0.12