ImageBundle uses depracated way to obtain service from container in Symfony 4.0

[bedrich-schindler]

When codeception is runned on our projects, it throws following notice:

The "assets.packages" service is private, getting it from the container is deprecated since Symfony 3.2 and will fail in 4.0. You should either make the service public, or stop using the container directly and use dependency injection instead.

Deprecation notices cause that codeception tests fail. More info about this problem: https://symfony.com/blog/new-in-symfony-3-4-services-are-private-by-default

[Sentence]

Pull request (#111) created, it works for me, but need a review from maintainers.