Add vagrant-vbguest plugin

https://github.com/dotless-de/vagrant-vbguest

Technically, whenever a user spins up a new basebox, they should make sure that the [Virtualbox Guest Additions](http://www.virtualbox.org/manual/ch04.html) package in the VM matches the version of virtualbox that they're running on their host machine. Baseboxes always come with one installed, but the basebox vbguest version is rarely as up-to-date as your host vbox version, so they're not in sync.

We can use this plugin to keep them in sync. It does a quick check on the version on each `up`/`reload`, and only runs when needed (it can take a little bit of time).