Fix tags config

Fix "PHP Notice:  Undefined index: tag_listener in friendsofsymfony/http-cache-bundle/DependencyInjection/FOSHttpCacheExtension.php
on line 73" when ExpressionLanguage is not available.

A problem here is that we only test on Travis with ExpressionLanguage installed, so we're not able to check all config cases. We may want to add tests where we don’t install ExpressionLanguage to check whether our config is compatible with Symfony LTS 2.3 (from 2.4 onwards ExpressionLanguage ships with Symfony full-stack).