Bump required cmake version from 2.x to 3.x (#801)

diff --git a/CMakeLists.txt b/CMakeLists.txt
index 5797f87..856bc98 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -1,4 +1,4 @@
-cmake_minimum_required (VERSION 2.8.12)
+cmake_minimum_required (VERSION 3.5.1)
 
 foreach(p
     CMP0048 # OK to clear PROJECT_VERSION on project()