[#64650014] Changes to support Hiera eYAML GPG

__DEPENDS ON https://github.gds/gds/puppet/pull/2149__

Use Vagrant box version `20140919`, which includes the `ruby-hiera-eyaml-gpg` package.

Also, ensure that the `tools/copy-hiera-gpg-keys` script added in [gds/puppet#2149](https://github.gds/gds/puppet/pull/2149) is called to copy any test GPG keys (used for testing encrypted Hiera data under [alphagov/vagrant-govuk](https://github.com/alphagov/vagrant-govuk)) to `/etc/puppet/gpg` so that Puppet is able to decrypt the Hiera data.