Automatically generate updateinformation when being run on Travis CI

We should encourage and automate as far as possible the generation of updateinformation.

If the environment variable `$TRAVIS_REPO_SLUG` is set (which in this example would be `AppImage/AppImageUpdate`), then automatically generate the string `-u "gh-releases-zsync|AppImage|AppImageUpdate|continuous|AppImageUpdate-*x86_64.AppImage.zsync"`.

At least advertise it, better simply use it unless specifically deactivated.