v2.2: Added Laravel 5.8 / PHP 7.3 support (#10)

* Updated composer.json to work with Laravel 5.6

* Updated PHPUnit compatibility

* Laravel 5.7 compatibility

* Laravel 5.7 support

* Fix merge

* Added PHP 7.3 travis check

* Added Laravel 5.8 support

* Fix PHPUnit void return type declaration issue

* Fix attribute warnings of newer PHPUnit