Support latest Desktop Entry Specification (1.1)

I run: `appimagetool-x86_64.AppImage "AppImage"`

I get the error:
```
appimagetool, continuous build (commit d185526), build 1855 built on 2018-10-07 20:18:45 UTC
WARNING: appstreamcli command is missing, please install it if you want to use AppStream metadata
/home/tastytea/Projekte/whyblocked/build/AppImage/de.tastytea.Whyblocked.desktop: error: value "1.1" for key "Version" in group "Desktop Entry" is not a known version
/home/tastytea/Projekte/whyblocked/build/AppImage/de.tastytea.Whyblocked.desktop: warning: key "Keywords" in group "Desktop Entry" is a reserved key for KDE
ERROR: Desktop file contains errors. Please fix them. Please see
https://standards.freedesktop.org/desktop-entry-spec/latest/
for more information.
```

According to the specification that the error message points to, `Version=1.1` is perfectly fine and `Keywords` is not reserved.