Enable Travis CI for PostgreSQL

Setting up Jenkins tests for PostgreSQL is not a priority for WMF, so
this change enables Travis CI tests run against a PostgreSQL database.

At the moment, those tests will fail because various fixes for
PostgreSQL issues are still pending review.  However carefully testing
individual commits (cf. for example
https://travis-ci.org/scfc/mediawiki-core/builds/40319020) shows that
the test setup itself is working as expected.

Travis CI's hhvm does not support PostgreSQL, so for now this change
only tests PostgreSQL with PHP 5.3.

Also the range of branches that tests are run against is widened to
include travis-ci/*.  This allows developers to enable Travis CI in
their GitHub fork of wikimedia/mediawiki and then push to such a
branch without having to mess up the master or wmf/* branches with
their defined meaning.

Bug: 37602
Change-Id: I9c2efa3fbc589d2d4198120ca80501124b1b16b9