use nfs for better i/o performance on the synced directory

Use nfs for the synced ```/vagrant``` directory to speed up i/o.

This improved performance ~10% when running all activerecord tests on my 2011 Macbook Air. There are a couple of downsides to consider, not sure if these are dealbreakers:
* Will not work in Windows
* Requires to you enter your password for root privileges when starting the VM