Make maintenance/update.php parse again under PHP 4.1.0

The instanceof produces a parse error on old PHP versions,
which doesn't show the old-version error message.

Note that our is minimum PHP version is 5.3.2, so it won't
produce E_STRICTs (is_a() was undeprecated in 5.3.0)

Change-Id: I5904301759bd154dd87ed90a5176369e37346482
Merge "Make maintenance/update.php parse again under PHP 4.1.0"