Supply a valid AppData ‘content_rating’ to fix #26

[musicinmybrain]

Validation with appstreamcli reports that this element must not have text content, which in this case was just none anyway.

After this PR:

$ appstreamcli validate --no-net --explain data/net.sourceforge.wavbreaker.appdata.xml
I: net.sourceforge.wavbreaker:9: description-first-word-not-capitalized
   The description line does not start with a capitalized word, project name or number.

I: net.sourceforge.wavbreaker:~: content-rating-missing
   This component has no `content_rating` tag to provide age rating information. You can generate
   the tag data online by answering a few questions at https://hughsie.github.io/oars/

✔ Validation was successful: infos: 2

[musicinmybrain]

$ appstreamcli validate --no-net --explain data/net.sourceforge.wavbreaker.appdata.xml
I: net.sourceforge.wavbreaker:10: description-first-word-not-capitalized
   The description line does not start with a capitalized word, project name or number.

✔ Validation was successful: infos: 1

[thp]

Thanks!