Solve deprecation warning generated by git safeclone

[traversaro]

CMake Deprecation Warning at /usr/local/src/robot/robotology-superbuild/build/src/CppAD/CMakeFiles/YCMTmp/CppAD-gitsafeclone.cmake:8 (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.

[traversaro]

Fixed by #475 .