Vagrant/Virtualbox - Using host resolver as a DNS proxy

This fixes a potential issue where the vm guest cannot resolve DNS on its own, observed when the VM is started and the host subsequently connects to a VPN.

Fix was found by @weeirishman:

> I've actually just reloaded the VM after reading the following:
> http://www.virtualbox.org/manual/ch09.html#nat_host_resolver_proxy
> 
> I added this to the .vagrantfile:
> vb.customize ["modifyvm", :id, "--natdnshostresolver1", "on"]