Added --prefer-lowest tests

This ensures that the dependencies constraints are not only valid for the latest versions specified but also for the lowest versions allowed. Only running on 5.4 since that is the "main" version of Laravel, at least the oldest you can have.

(Will squash when it passes – or fails on lowest at least)
Because you want to know if it works *both* with the oldest and latest, bugs might be introduced in latest, while you might unknowingly rely on methods and things that aren't present in oldest versions.