Update isl.rb

Updates isl to latest version 0.13.

There's a fairly significant problem with the current isl on Homebrew in that the link it uses to get the package is stone-cold dead, along with the entire website where the package came from. I have found the best solution I could which is to tap the latest stable release tar.gz from the git and changed the configure procedure accordingly. 

Hopefully this passes the built bot and doesn't cause any problems. The vanishment of the stable release website we previously tapped for tar.gz links is a concern.
I think this works: http://repo.or.cz/w/isl.git/snapshot/isl-0.13.tar.gz