Fix EZP-21631: RoutableFragmentRenderer::generateFragmentUri() not compa...

...tible with Symfony 2.3.5

https://jira.ez.no/browse/EZP-21631

Related to https://github.com/symfony/symfony/commit/91234cd89aed6c5e5d6ec5947d2099ed787b6392 and https://github.com/symfony/symfony/issues/8458 .