Update min CMake version to 3.5

Building with a relatively up-to-date CMake results in the following warning:

CMake Deprecation Warning at CMakeLists.txt:19 (cmake_minimum_required):
    Compatibility with CMake < 3.5 will be removed from a future version of
    CMake.

    Update the VERSION argument <min> value or use a ...<max> suffix to tell
    CMake that the project does not need compatibility with older versions.

We don't use older CMake features and can update cmake_minimum_required