Updated dependencies break deployment

The Docker dependency was updated in the Berkshelf file, but not in the metadata.rb, causing deployments to fail.

Updating the Docker dependency caused a version inconsistency between the nginx and all other cookbooks on the dependency on build-essential (~> 1.4.4 versus >= 0.0.0), since build-essential v2.0.0 is now available. This addresses both issues, with these changes I'm able to deploy using the current master (a7f76c688c6b0f4b49b85afb89af99037a58abe6) version of the Dokku cookbook again.