Add missing RedirectRoute::getChildren method

If I add a ``Route`` in a ``RedirectRoute`` it works, but the tree won't show the children, because of the missing ``getChildren()`` method. This happens [here](https://github.com/sonata-project/SonataDoctrinePhpcrAdminBundle/blob/master/Tree/PhpcrOdmTree.php#L298) : an exception is thrown and the ``$prop`` variable is ``null``.