Update travis build to use Symfony 2.8.3 or later
Update travis build to use Symfony 2.8.3 or later

To be in sync with composer.json dependencies, which also require Symfony 2.8.3 or later.
Required to fix You have requested a non-existent service "fos_user.doctrine_registry" error during CI build.
joschi127/doctrine-entity-override-bundle,joschi127/doctrine-entity-override-bundle