Vagrant VM fix: don't mount filesystem using NFS in windows

Vagrant NFS mounting doesn't work under Windows, hence this fix.

The code here uses [a more robust way to determine a Windows system](http://stackoverflow.com/questions/4871309/what-is-the-correct-way-to-detect-if-ruby-is-running-on-windows) than d869d840a78dacb0312c3672b0e8ca40f0810752.