Enable vagrant-cachier if available

This dramatically speeds up provisioning times. Enable the plugin if
installed. It can be installed using Boxen. I'm sure that we did this once,
but I can't find any record of it. However it now seems to work better with
Lucid.

This repo doesn't currently support VMware so I'm not going to bother
feature flagging it or wrapping it in a provider block.
Merge pull request #15 from alphagov/enable_cache_plugin

Enable vagrant-cachier if available