Fix ordering of postgres classes

The postgres lib devel class must not be included until after our
postgres primary class, as it inherits from postgresql::globals, and we
need to change the paramaters of this from within pp_postgres_server

Added an explicit ordering relationship within the class as well rather
than just relying on parse order.
Good call, I tried this way round and it turns out my ordering was not working as expected. I've now updated it to add a new ordering which seems to work correctly...