Define specific buildpacks and versions in Chef

With this PR we can now customize the buildpacks that are included in Deis and provide specific revisions for each buildpack repository for fine-grained control during releases.

Note that the repositories are synchronized on every `chef-client` run.  If a buildpack is pointing to `master`, for example, it will be upgraded continuously.

Also, for reasons unknown the Ruby buildpack writes to the buildpack directory, forcing us to mount it read/write. :(