Update Boxen to support OSX Yosemite (10.10)

In order to support Yosemite, the `boxen` gem needs to be version `~> 2.7.0`.

This PR merges in all the changes since our last pull from upstream so that we are close to upstream and ready for the Yosemite release (yesterday).

My manifest was causing me issues, so I've amended it. I doubt anyone else will have issues as they don't use `vagrant_manager` or don't try to override the boxen git version.