Label `ci-slave-5` as having Postgres 9.3

This means we'll be able to tie builds that depend on Postgres 9.3
features to any slave with this label, rather than to a specific slave.

From `hieradata/role.ci-slave.yaml`, it appears that the MongoDB version
is pinned, so we can safely have that label. I'm not so sure about the
`rabbitmq` and `java6` labels, so I've left them out.

Ideally, these labels would be automatically generated from what's
actually provisioned on the servers, but I figured that would be opening
up a whole new can of devops.
Merge pull request #251 from alphagov/label-slave-5-postgres-version

Label `ci-slave-5` as having Postgres 9.3