Mount GPG keys (for testing) in the guest VM

Mount the `gpg/` directory in the Puppet repo on the host machine to
`/etc/puppet/gpg` on the guest VM.

This is necessary so that Puppet can decrypt GPG-encrypted Hiera data as
necessary for testing in a `vagrant-govuk` VM.
Upgrade Vagrant box to include Hiera eYAML GPG

Upgrade the Vagrant box used by this repo to version `20140919` that
includes the `ruby-hiera-eyaml-gpg` APT package (which is consitent with
how Puppet, Hiera and Facter are provided to dev VMs).