Repackage base boxes, used by rspec tests

This allows us to more easily repackage and republish base boxes used in the
rspec test suite.
I think it's pointless. If you'd like to feel the pain of doing this portably, check: https://github.com/thoughtbot/rcm/blob/master/configure.ac#L9-L15

But for the laptop repo: `/bin/sh` is perfect.