Bump Vagrant to 1.5.3

1.4.x is broken - the download URL changed after 1.4, which got broken on
this change:

https://github.com/boxen/puppet-vagrant/commit/ef3921a8db039407e03f3e67443ae57dd31316f1#diff-60ae41fd0a31977447947f59940ee9a4L10

and 1.5.3 is the last stable upgrade pre-1.6.0.

Given Vagrant is often unstable early in development cycles I thought it
prudent to pin it to the last previous version rather than 1.6.x latest.

Fixes #91
Merge pull request #92 from alphagov/vagrant-153

Bump Vagrant to 1.5.3