Fix psql drop constraint issue on primary keys

This attempts to fix an issue when dropping a primary key in PostgreSQL.

Currently, the code handles foreign key dropping fine (by dropping the `tablename_fieldname_fkey` constraint), but ignores the case of primary keys (`tablename_pkey` constraint).  This adds logic to handle that.

I have tested this manually and it fixes the issue I was having using `DBIC::DeploymentHandler` to generate a migration that changes the primary key column on a table.  Additionally, all existing tests in `SQL::Translator` pass with this change.

I have added some very simple tests, based around the FOREIGN_KEY tests I found in `t/47postgrs-producer.t`, and I'd love some feedback on those as well.  Are there other places I should be adding tests?