Update Kid3.app to v3.1.1

This url should be left alone. `#{version}` ensures it’ll be kept to date for the foreseeable future, and it still works. In addition, we have [sctrict rules regarding sourceforge urls](https://github.com/caskroom/homebrew-cask/blob/master/CONTRIBUTING.md#sourceforge-urls), and the one you’re trying to change it to, breaks the build.