Require at least Vagrant 1.2.x in Vagrantfile

Users don't read READMEs or documentation, they'll
just fire 'vagrant up'. Vagrant already checks the
Virtualbox version itself, but for the Vagrant version
no checks exist. Vagrant 1.4 provides a method 'require_version'
which is pretty useless for older versions, like on Debian
Wheezy (1.0.3). So we'll ship our own version comparison.

fixes #6366