Add caching support

`vagrant destroy ---force && vagrant up` should be as painless as possible, but on a slow connection, all the packages need to be redownloaded. This is also an issue for gems, and to a degree, drush.

Ariadne did it via a messy shared_folder in the Vagrantfile:
https://github.com/myplanetdigital/vagrant-ariadne/blob/develop/Vagrantfile#L74-L75

`vagrant-cachier` seems to solve it in an elegant way:
https://github.com/fgrehm/vagrant-cachier

It currently uses native vbox shares, which are slow on vbox, but there's a recent issue aiming to resolve this:
https://github.com/fgrehm/vagrant-cachier/issues/7

I imagine we could also write a plugin to add support for a drush "bucket" (in vagrant-cachier parlance), but not sure if that's worth it. These would be the places we'd need to extend:
https://github.com/fgrehm/vagrant-cachier/tree/master/lib/vagrant-cachier/cap/linux
https://github.com/fgrehm/vagrant-cachier/tree/master/lib/vagrant-cachier/bucket
https://github.com/fgrehm/vagrant-cachier/blob/master/lib/vagrant-cachier/plugin.rb#L11-L14