Updated Gem dependencies

- No longer rubocop'ing the metadata.rb.
- Updated Berksfile for Berkshelf 3.0.
- Updated Rakefile so 'rake test' doesn't automatically test-kitchen
Switched from Vagrant as default to Docker

Switched to using kitchen-docker-api instead of kitchen-vagrant by default. Check out @fnichol's awesome dvm if you need a quick Docker host.