Update to isl 1b3ba3b72c0482fd36bf0b4a1186a259f7bafeed

This includes the following very useful isl commit:

commit d962967ab42323ea5ca0398956fbff6a98c782fa
Author: Sven Verdoolaege <skimo@kotnet.org>
Date:   Wed Dec 18 12:05:32 2013 +0100

allow the user to impose a bound on the number of low-level operations

This should allow the user to deterministically limit the effort spent on a
computation.