Lock Berkshelf to v3.1.5
Only run unit tests in ChefDK verify

The Berkshelf cucumber tests are failing because they make assumptions 
about latest versions of cookbooks on Supermarket. The `berkshelf-api` 
cookbook received an update today that broke the tests!
Merge pull request #242 from opscode/schisamo/lock-berks-down

Lock Berkshelf Down; Only run ChefDK unit tests