Remove version lock on chef-docker

Now that we're using Docker 0.9.0, we should resume using the latest
chef-docker release. I locked this to 0.31 because 0.32 introduced
breaking changes for those still using Docker 0.8.0.
We don't need Berksfile.lock
Merge pull request #96 from opdemand/carmstrong/remove_chef-docker_lock

Remove version lock on chef-docker