Pin (and unpin when fixed) postgresql module in acceptance tests

The 5.2.0 release of the postgresql module included a minor ordering regression. Unfortunately, the puppetdb module was affected by this. Consequently, we pinned the postgresql module to 5.1 prior to installing the puppetdb module in our acceptance tests. here for 5.1.x+ and here for 2.x.

With the release of postgresql 5.2.1 this is supposed to be fixed. We should attempt to unpin, test that it now works, and update our tests.

Original failing job link: https://jenkins-master-prod-1.delivery.puppetlabs.net/job/platform_puppetserver_integration-system_no-conditional_smoke-2.x/17/