Fix NFS configs for 1.4

This is the fix for GH-67 but as there are some open issues related to NFS with VirtualBox on Vagrant's core I can't really be sure that things are working fine (I think the one that consolidates them is https://github.com/mitchellh/vagrant/issues/2674). But since I was able to reproduce the original bug, this should be enough to fix things from our side and will allow people to use Vagrant < 1.4 as well.

@tuminoid would you mind [installing the plugin from sources](http://fgrehm.viewdocs.io/vagrant-cachier/development) out of this branch and double checking that it works fine on vmware? If you need any help with that please LMK.

/cc @phinze