cmake: set minimum version to 3.5

--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -19,7 +19,7 @@
 #       start libevent.sln
 #
 
-cmake_minimum_required(VERSION 3.1 FATAL_ERROR)
+cmake_minimum_required(VERSION 3.5 FATAL_ERROR)
 
 if (POLICY CMP0054)
     cmake_policy(SET CMP0054 NEW)