I'm getting a lot of deprecation notices since #16101: Passing an instance of Symfony\Component\Validator\ValidatorInterface as argument to the Symfony\Component\Form\Extension\Validator\EventListener\ValidationListener::__construct method is deprecated since version 2.8 and will be removed in 3.0. Use an implementation of Symfony\Component\Validator\Validator\ValidatorInterface instead: 1335x, see Travis log (ignore the other notices).

Since I'm not using the validator directly, I guess some code of Symfony itself needs to be updated to get rid of them.