Example appdata.xml is doesn't validate with appstream-util validate, lacks required `<translation>` and `<summary>` tags

The example appdata.xml generated by latest AppImage is invalid and won't validate with appstream-util even when filled out properly: it lacks required `` and `` tags.

Edit: the appstream-util version I used is 0.7.1 as shipped with Fedora 26

Edit: I just noticed appimagetool itself might actually not require those, but due to #450 I'm required to guess based on `appstream-util` what sort of error might be the problem..