Update libigl.cmake

cmake/libigl.cmake

@@ -1,4 +1,4 @@
-cmake_minimum_required(VERSION 3.1)
+cmake_minimum_required(VERSION 3.8)
 
 # https://github.com/libigl/libigl/issues/751
 # http://lists.llvm.org/pipermail/llvm-commits/Week-of-Mon-20160425/351643.html