Implement getDefaultDriver

This is a required interface in L4.2 since https://github.com/laravel/framework/commit/8b523557560765d19e67e190c7272c6634f3fdc4

Will result in a fatal PHP error otherwise:
> PHP Fatal error:  Class Basset\Factory\FactoryManager contains 1 abstract method and must therefore be declared abstract or implement the remaining methods (Illuminate\Support\Manager::getDefaultDriver) in vendor/etrepat/basset/src/Basset/Factory/FactoryManager.php on line 5