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)
```yaml
install:
  - if [ "$dependencies" != "lowest" ]; then composer install -n; fi;
  - if [ "$dependencies" = "lowest" ]; then composer install --prefer-lowest --prefer-stable -n; fi;
```

I know that code doesn't work, but do you see the point? Also, you're missing `--prefer-source` from your commands.