The symfony Symfony 2.8.18 release contains a test that requires phpunit 6.0 which only is only available for PHP 7. Therefore, executing the unit tests with still supported PHP 5.6 is impossible because of this error:

PHP Fatal error:  Class 'PHPUnit\Framework\TestCase' not found in /home/travis/build/aimeos/aimeos-symfony/vendor/symfony/symfony/src/Symfony/Bundle/FrameworkBundle/Test/KernelTestCase.php on line 25

Full stack trace can be found at https://travis-ci.org/aimeos/aimeos-symfony/jobs/208945815