Use vagrant-vbguest to make sure the guest addition matches the vbox version running

It's not always the case that it matters that the guestadditions version in the VM matches the host's version, but a mismatch has been the source of many quirky failures on the vbox end of things.

https://github.com/dotless-de/vagrant-vbguest

(The travis-ci project recommends it for working with the baseboxes they offer, if that helps convince.)