Upgrade to Berkshelf 3.

When generating a new cookbook with Meez the Gemfile only specifies Berkshelf without a version number.    A bundle install will pull down the latest Berkshelf which is version 3.x which has slightly changed the sources are defined in the Berksfile.

Berkshelf 3 is a decent step forward so is good to use anyway, however the main problem without upgrading is that a `berks install` will complain due to the old style Berksfile.