Support Laravel 5.7.*

[usamaejaz]

As the title suggests, the installation fails for laravel 5.7.*. Migration of 5.6 to 5.7 has only a few changes required. It may not even require any change for this package though I am not sure.

[usamaejaz]

I created this pull request as I needed this to be fixed quickly for me. #8

[calebporzio]

Thanks for the PR!