Raise CMake minimum required version

[rndxelement]

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.

This holds for CMakeLists.txt, vendor/alac/CMakeLists.txt and vendor/docopt/CMakeLists.txt.
Issue at docopt: docopt/docopt.cpp#166