Too many redundant tests are run on Travis

Currently we're running 18 tests per commit. Each test takes about 20 minutes.

Travis only allows a limited number of simultaneous builds (and we already have many more than they typically allow for free). This means you may need to wait several hours for tests to catch up to your PR.

Our matrix right now tests 5.3.3, 5.3, 5.4, 5.5, 5.6, hhvm * mysql, mariadb, and postgresql.

We really only need to test each PHP version on ONE DB (mysql), then test other DB types once on one PHP version.

Other than PHP bugs, there should be no difference between running the postgres or mariadb tests once versus all PHP versions (the DB versions should all be the same for every PHP version).

Doing this means we can reduce our builds from 18 to 8 and we should not be adversely affected by it.