Fix for test build/fail regression

Turns out the test improvements pull request (#6) caused a regression where test failures didn't signal build failures.  This is because drush 6.x doesn't return the correct exit code.  7.x does, but it's not stable enough yet.  Unfortunately, drush 5.x isn't available via Composer/Packagist, so we have to revert to the less reliable pear install method.