[#82935274] Upgrade Vagrant Puppet version to 3.7.1 from 3.0.2

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These are no longer actively maintained (last commit was two years ago)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 PR 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.
Thanks @bradleywright and @alext.

Summary: From discussion IRL, we prefer to be explicit about the version in the Gemfile because:

- it prevents bundle from opportunistically upgrading gems when upgrading dependent gems (and we never want to edit `Gemfile.lock` by hand)
- it makes it clearer to humans what version we're expecting
- it avoids people reviewing PRs from having to check `Gemfile.lock` manually, which is hidden by default on Github

I've rebased to amend the `Gemfile`.