Fix Vagrant file on Ubuntu with extra VM config

On the following two Ubuntu systems, the "vagrant up" command failed
with an issue related to symlinks in Shared folders.

- Ubuntu 12.04 with vagrant-1.3.5 and VirtualBox 4.1.12
- Ubuntu 13.10 with vagrant-1.3.5 and Virtualbox 4.2.16

See the following discussion and fix:
- https://github.com/rodjek/librarian-puppet/issues/57
- https://github.com/mitchellh/vagrant/issues/713

@tombooth has tested that this doesn't adversely affect VirtualBox on Mac.