isl: add latest version (#2834)

added latest version of isl. Needed to get llvm/clang in the latest
trunk to build via spack, but there are more versions available.
what is the policy regarding those? Add what we need for now?

var/spack/repos/builtin/packages/isl/package.py

@@ -31,6 +31,7 @@ class Isl(Package):
     homepage = "http://isl.gforge.inria.fr"
     url      = "http://isl.gforge.inria.fr/isl-0.14.tar.bz2"
 
+    version('0.18', '11436d6b205e516635b666090b94ab32')
     version('0.14', 'acd347243fca5609e3df37dba47fd0bb')
 
     depends_on("gmp")