Change Vagrant box; upgrades Puppet to 3.7.1
Change Vagrant box; upgrades Puppet to 3.7.1

Previously, we were using these Vagrant boxes from Puppet Labs:
https://github.com/puppetlabs/puppet-vagrant-boxes

These are no longer actively maintained (last commit was two years ago)
and supply an old version of Puppet, version 3.0.2.

This commit switches to use the new Puppet Vagrant boxes built using
Packer:
https://github.com/puppetlabs/puppetlabs-packer

No URL is necessary as these boxes are downloaded from Vagrant Cloud:
https://vagrantcloud.com/puppetlabs

This also allows us to use the new Ubuntu Trusty box from Vagrant Cloud
while keeping the Puppet version consistent between Precise and Trusty.

I have tested `ci-master-1`, `ci-slave-1`, `ci-slave-2` and
`transition-logs-1` using this new Vagrant box.

boffbowsh/ci-puppet,alphagov/ci-puppet,alphagov/ci-puppet,boffbowsh/ci-puppet,boffbowsh/ci-puppet,alphagov/ci-puppet,alphagov/ci-puppet,boffbowsh/ci-puppet