Fix teardown. Closes #2332

Basically just made the same change to sails-postgresql as made to sails-memory [in this commit](https://github.com/balderdashy/sails-memory/commit/545f61acb3cfe3eca01c44370414e36ce0a067e3), and discussed in Sails issue [#1632](https://github.com/balderdashy/sails/issues/1632).

This should close Sails issue [#2332](https://github.com/balderdashy/sails/issues/2332).