Add a build to the test matrix using the 'unstable' repo

[nmaludy]

Would be nice to have a permanent build in the test matrix that runs against the 'unstable' PackageCloud repo. This would help with testing and verification for changes that are merged upstream but not yet released.

Should be able to specify hiera data to make this happen: https://github.com/neillturner/kitchen-puppet/blob/master/provisioner_options.md