Fixed Application::loadDeferredProvider() expecting a service name but instead given the service provider class.

Had a problem using `$app['artisan']->call()` which triggers `Application::loadDeferredProviders()`. The problem what loadDeferedProvider should receive the instance name (`$service`) and not the service provider class name (`$provider`) which is what happening at <https://travis-ci.org/orchestral/testbench/jobs/12097378> 

Same method works fine in 4.0