Fix `make book` on Mac

The BSD version of sed included on Mac OSX has very primitive argument parsing. When running `make book`, `fix-edit-button.sh`'s sed call fails because the `-i` flag tries to associate with the sed script. My fix checks if `--version` succeeds to decide if it should use the BSD or GNU syntax.
A developer that uses mac is most probably going to have Brew or MacPorts, so no for the first question. And I believe that any user that encouters this error have the presence of mind to fix it for himself.