CMakeLists.txt: bump CMake version to 3.15

CMakeLists.txt: bump CMake version to 3.15

This change fixes the following warning on the latest CMake versions:

CMake Deprecation Warning at CMakeLists.txt:1 (cmake_minimum_required):
  Compatibility with CMake < 3.10 will be removed from a future version
  of CMake.

  Update the VERSION argument <min> value.  Or, use the <min>...<max>
  syntax to tell CMake that the project requires at least <min> but has been
  updated to work with policies introduced by <max> or earlier.