#18515 should do it now

The failure seems to be related to the changes from #18272 (only the builds for the master branch export the SYMFONY_PHPUNIT_OVERLOAD that's also the reason why the builds didn't fail on my fork as I used a separate branch for testing) when being used with PHPUnit 4.8 (PHP 5.5 is the only job not using PHPUnit 5). Note that I disabled exporting the variable which made the bridge register more stuff. Though I am currently not sure how to fix this best (but we for sure don't seem to have to update the Filesystem component).