Don't require PHPUnit at all

Author: matthiasnoback

composer.json

@@ -35,7 +35,6 @@
         "symfony/framework-bundle": "~2.8|~3.0|~4.0",
         "symfony/validator": "~2.8|~3.0|~4.0",
         "symfony/yaml": "~2.8|~3.0|~4.0",
-        "phpunit/phpunit": "^6.0",
         "symfony/security": "~2.8|~3.0|~4.0"
     }
 }