when on symfony 3.4 or better, we should support the `immutable` instruction in the cache rules: http://symfony.com/blog/new-in-symfony-3-4-http-immutable-responses
when on symfony 3.4 or better, we should support the
immutableinstruction in the cache rules: http://symfony.com/blog/new-in-symfony-3-4-http-immutable-responses