Ensure Vagrant version is at least 1.8.0
Ensure Vagrant version is at least 1.8.0

Use the Vagrant.require_version helper documented here:
https://www.vagrantup.com/docs/vagrantfile/vagrant_version.html

Eliminates issues like #238.

ALikhachev/saltfs,UK992/saltfs,rwaweber/saltfs,servo/saltfs,ALikhachev/saltfs,ALikhachev/saltfs,UK992/saltfs,servo/saltfs,setupminimal/saltfs,UK992/saltfs,setupminimal/saltfs,setupminimal/saltfs,rwaweber/saltfs,rwaweber/saltfs