Bump minimum cmake version, this builds with no warnings
beyond request to bump to 3.10
https://bugs.gentoo.org/951811
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -1,4 +1,4 @@
-cmake_minimum_required(VERSION 2.8.1)
+cmake_minimum_required(VERSION 3.5)
 project(UnitTest++)
 
 option(UTPP_USE_PLUS_SIGN