Specify correct return type for `Pdo\Connection::getLastGeneratedValue`

See http://php.net/manual/en/pdo.lastinsertid.php for reference that it returns `sting` instead of `int`.

TODO: change type of `$lastInsertValue` property and `getLastInsertValue` method in `Zend/Db/TableGateway/AbstractTableGateway.php` accordingly.