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.
Yeah. It is definitely possible, my initial fix on my machine was to update `sed`. I just wanted to fix the script for other Mac users. But I'm starting to believe it's easier to just leave it as is. Should I close the issue then?