Update links in README.md for C extension.
Update links in README.md for C extension.

SketchUp/testup-2,thomthom/testup-2,thomthom/testup-2,SketchUp/testup-2,SketchUp/testup-2,thomthom/testup-2,SketchUp/testup-2,SketchUp/testup-2,thomthom/testup-2,SketchUp/testup-2,thomthom/testup-2,thomthom/testup-2