Update vagrant box to 20140829

I have updated our Vagrant boxes to 20140829, which is based on Ubuntu
12.04.5 with the latest lts-trusty kernel. We should use this.

I've tested it - the jumpbox comes up fine (it's already been tested
against the full dev VM).
Merge pull request #30 from alphagov/update-vagrant-boxes

Update vagrant box to 20140829