[8.x] Remove TrustHosts reliance on config helper

[axlon]

This PR removes the TrustHosts middleware's reliance on the config() helper, this achieves two things:

• Developers can now unit test their application's implementation more easily, because they can simply mock the application instance this middleware receives through it's constructor
• If a developer were to extend Illuminate\Contracts\Foundation\Application::environment() for some reason, the middleware would now take that into account

This change should be BC, except when \Illuminate\Contracts\Foundation\Application::environment has been extended. In which case the current check already didn't work and likely was overwritten at an application level anyway.

[GrahamCampbell]

Seems like a sensible change to me. 👍

[GrahamCampbell]

Maybe for 9.x though. People manually changing the config value in their tests will have stuff broken.

[axlon]

@GrahamCampbell I'll remake the PR to master 👍