disable postgres tests by default

Just a suggestion, but since I needed it anyway I made a pull-request out of it:

Let's give the option to enable and disable platform/database specific tests. Why? For me, it's just that I have a different connection string for postgres. We could make it possible to pass it in, however, someone else might not have postgresql installed. As long as the platform-specific tests are run on travis before merge, I think it's more intuitive (for new developers) if the tests pass right away and no extra setup is required.

Let me know what you think.