[CMake] update minimum version

Recent CMake versions added this warning:
CMake Deprecation Warning at CMakeLists.txt:6 (CMAKE_MINIMUM_REQUIRED):
  Compatibility with CMake < 2.8.12 will be removed from a future version of
  CMake.

And we are not compatible with CMake < 3.1:
ref https://github.com/jrl-umi3218/jrl-cmakemodules/issues/338