DO NOT MERGE: Updating Travis to use develop branch of support module.

Relies on https://github.com/silverstripe-labs/silverstripe-travis-support/pull/6 being merged first.

This will check that we're okay to run PHPUnit 4, which has been updated
in the develop branch of the silverstripe-behat module. It's a preliminary
check to make sure we're okay to merge https://github.com/silverstripe-labs/silverstripe-behat-extension/pull/54