[Generators] ActiveJob

WIP !
Actually, we don't need this feature check at all here. Not unless we don't load the generator unless we check the feature too. All the generator will do is create the spec file. It doesn't create the actual job object. So, there's no real dependency with Rails here. If we want this only to be available if `ActiveJob` is, then the generator itself should not be loaded unless `ActiveJob` is loaded.