CMake: Bump mininum version requirement to 3.5
Silences spam such as the following during CMake configure:
CMake Deprecation Warning at CMakeLists.txt:2 (cmake_minimum_required): Compatibility with CMake < 2.8.12 will be removed from a future version of CMake.
Update the VERSION argument value or use a ... suffix to tell CMake that the project does not need compatibility with older versions.
Even xenial has cmake 3.5.1: https://launchpad.net/ubuntu/xenial/+source/cmake