Only enable postgresql mirror for trusty
Only enable postgresql mirror for trusty

We don't need the mirror on precise machines, and *if* we add Xenial
machines in the future the version of PostgreSQL we need the mirror for
is available by default, so only enable it for trusty machines.

alphagov/govuk-puppet,alphagov/govuk-puppet,alphagov/govuk-puppet,alphagov/govuk-puppet,alphagov/govuk-puppet,alphagov/govuk-puppet