[5.7] Explicitly list whoops as a suggested dependency

[GrahamCampbell]

No description provided.

[browner12]

It's already in our 'require-dev'. Why are we suggesting it?

[thecrypticace]

Because a package's require-dev doesn't propagate to a project's require-dev. Now we'd be saying "please add whoops if you want more helpful error pages in development"

It is already in the require-dev of laravel/laravel but if someone were to pull the laravel/framework package separately they wouldn't necessarily have that.