On Travis-CI, panopoly_test should be enabled AFTER completing the upgrade

Right now, during upgrade tests on Travis-CI, we're attempting to enable panopoly_test BEFORE the upgrade, which causes it to download the latest -dev of panopoly_test from Drupal.org. So far, this actually works most of the time! But it really should instead be using the version of panopoly_test that was built with the latest Git, which would be the case if it enabled panopoly_test AFTER completing the upgrade.