Release 12.0.0 alpha 0

This release does not contain most of the changes planned for the 12.0 release,
but it does have a few incompatible changes that necessitate a version bump.
Since ChefDK's appbundler feature doesn't support git-installed gems and the
chef-dk library relies on some fixes in the Chef 12 code base, we are making a
very early rubygems-only prerelease.