Add CI testing of postgres support
test: Fix forcing the creation of a record with a given id.

The primary key can't be set using the attributes hash.  Postgres had a
problem with this because the id of the first row was set to a specific
value, which didn't count towards the id sequence, so it automatically
assigned 1 instead of MAX(id) + 1.
Merge pull request #188 from Shopify/pg-ci

Add CI testing of postgres support