Remove usage of vagrant_gem and plugin gem installs

- Everyone should be using the vagrant module which installs the
  official packages of Vagrant. No-one should be using the old and
  crappy gem install of vagrant.
- Vagrant plugins should be installed via the defined type, not compiled
  as gems for every ruby version on your system.
Merge pull request #57 from alphagov/remove-vagrant-gem

Remove usage of vagrant_gem and plugin gem installs