Fixed tests

[this](https://github.com/hannesvdvreken/eloquent-sluggable/blob/develop/tests/SluggableTest.php#L58) was no longer compatible with [that](https://github.com/orchestral/testbench/blame/master/src/Testbench/Traits/ApplicationTrait.php#L140) since 2 days.

I recommend using a tagged version of testbench.