allows extension of illuminate notification class

[Drewdan]

When developing using this package, I wanted to add a new notification type. So I created a new class an extened the \Illuminate\Notifications\Notification class however I received an error stating the new class I made had to extend the Illuminate\Notifications\Notification class, which my class was doing.

A workaround is extending the Spatie\FailedJobMonitor\Notification - however, the error message does not suggest this to be the correct source of action.

This pull request resolves the issue by importing the Illuminate\Notifications\Notification and uses it in the isValidNotificationClass method.

[freekmurze]

Thanks!