Add ability to short circuit trait booting

[dallincoons]

This is a very simple, easy tweak. The goal is to allow for doing things like testing individual methods on an Eloquent Model without needing to load the framework, since running a test that boots the Laravel framework is typically three times slower than one that doesn't.

Because bootAuditable immediately calls isAuditingEnabled, which references the App facade, you can't instantiate a model in a Laravel-free test without getting an error having to do with the facade not being available.

This tweak allows the user to simply add Article::$auditingDisabled = true; to the beginning of the test, and the problem goes away.

[anteriovieira]

Thanks @dallincoons

[cwilby]

Missed this the first time around on this issue, thanks @dallincoons