[1.0.13] Compatibility with PHPUnit ~6.0

[deleugpn]

This closes #99. Although it looks a breaking change, it actually isn't. If someone is overriding the setUp method, they're already calling the parent from within it otherwise the application doesn't work. This change only adds one more step to the setUp.

[dusk@php7 dusk-with-unit]$ php artisan serve &
[1] 2538
[dusk@php7 dusk-with-unit]$ Laravel development server started: <http://127.0.0.1:8000>

[dusk@php7 dusk-with-unit]$
[dusk@php7 dusk-with-unit]$ php artisan dusk
PHPUnit 6.1.0 by Sebastian Bergmann and contributors.

[Tue Apr 18 14:43:08 2017] 127.0.0.1:51566 [200]: /favicon.ico
.                                                                   1 / 1 (100%)

Time: 2.03 seconds, Memory: 10.00MB

OK (1 test, 1 assertion)