Simplify default driver lookup in DatabaseServiceProvider.php

As we discussed yesterday. Just makes it a bit more clear what we're doing here.

As a side note, I'm submitting a [pull request](https://github.com/laravel/framework/pull/6153) to Laravel to add a getDefaultDatabaseDriver() function to their database manager API. If it's accepted, we won't need this protected function anymore and we can just call `return new Dongle($this->app['db]->getDefaultDatabaseDriver(), $app['db']);`