use vagrant-cachier if present

[vagrant-cachier](https://github.com/fgrehm/vagrant-cachier) is a plugin which caches packages from various repositories for your vagrant boxes.

If you have it installed, we should use it. But we should break if you don't have it.

(You can [install it using boxen](https://github.com/alphagov/gds-boxen/blob/master/modules/people/manifests/philandstuff.pp#L37))