Workaround travis phpunit mixup

this should fix the build. 

the error on travis makes no sense whatsoever: https://travis-ci.org/symfony-cmf/CoreBundle/jobs/27420033 

PHPUnit 3.7.37 by Sebastian Bergmann.

PHP Fatal error: Call to undefined method PHPUnit_Util_Test::getHookMethods() in phar:///home/travis/.phpenv/versions/5.3.27/bin/phpunit/phpunit/Framework/TestCase.php on line 800

phpunit 3.7.37 has some different code on line 800. getHookMethods is phpunit 4 only. and travis seems not to be generally using a broken phpunit version - all other bundles still work as they should.