Migration lib update for easy use of MY_Migration

Update: removed the code from CI_Migration that skipped the __construct() execution when MY_Migration was present.

Reasoning:
* The user can avoid the original __construct() in MY_Migration by not calling parent::__construct() in his own constructor.
* On the other hand, an user with a non-significant MY_Migration library was forced to copy-paste CI_Migration::__construct() in his own constructor just to simulate the default functionality.