Avoid CMP0048 warning (#570)

Fixes #567.

If pybind's CMakeLists gets loaded via an include_directory from another
CMakeLists with a higher minimum version (e.g. 3.0), the project()
command without a version produces a CMP0048 warning.

This commit explicitly requests the new behaviour if the policy exists,
as it won't cause problems (we set VERSION later).