Remove support for old Vagrant config

Previously this Vagrantfile supported both versions 1 and versions 2 of the
Vagrant config. The version 1 config is for 1.0.x versions of Vagrant. However,
everyone now has to be using at least version 1.2.3 for Hiera support in GOV.UK
Puppet, so there is no longer any need to support version 1 of the config.