Preloading all step users fixes #7.

[lucaboesch]

No description provided.

[PhilippImhof]

Seems we did that at the same time :)

And we both forgot that preload_all_step_users() is not available before 3.11.

            if (method_exists($quba, 'preload_all_step_users')) {
                $quba->preload_all_step_users();
            }