Upgrade Vagrant box to use Puppet 3.6

Upgrade the Vagrant box versions in `Vagrantfile` to use the
latest box images which install Puppet 3.6 by default.

Without this, Puppet will upgrade itself to version 3.6 but will show
errors on the first run.
Merge pull request #32 from alphagov/79450194-puppet_3_6

[#79450194] Upgrade Vagrant box to use Puppet 3.6