Please update CMake version requirement

[Jellby]

CMake Deprecation Warning at CMakeLists.txt:1 (cmake_minimum_required):                                             
  Compatibility with CMake < 2.8.12 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.