bump minimum cmake version
Fixes the following warning with new CMake versions:
CMake Deprecation Warning at CMakeLists.txt:3 (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.