Upgrade to Vagrant 1.1 syntax.
Merge branch 'master' into vagrant_110
Merge branch 'master' into vagrant_110

Conflicts:
	Vagrantfile
Add backwards compat for Vagrant < 1.1

Based off the `vagrant_110` branch which has been running in parallel for a
long time. This brings the two together, maintaining support for older
versions. It's a bit nasty. But better than maintaining two branches or
completely separate code paths. Hopefully we'll phase out older Vagrant
installs soon.
Docs and deprecation warning for Vagrant version

Idea stolen from @bradleywright who appears to have been working on the same
thing in the gds/development repo.
Merge branch 'master' into vagrant_110
Merge branch 'vagrant_110' into vagrant_backwards_compat
Merge pull request #3 from alphagov/vagrant_backwards_compat

Add backwards compat for Vagrant < 1.1