Laravel 5.6 breaks the current version of the package

[AdrianHL]

One of the changes in Laravel 5.6 breaks the package https://laravel.com/docs/5.6/upgrade

The validate method of the ValidatesWhenResolved interface / trait has been renamed to validateResolved in order to avoid conflicts with the $request->validate() method.

Therefore the package needs to include a composer constraint to prevent using it with Laravel 5.6.