<release> versions are not in order does not respect dates

[ignatenkobrain]

Originally reported as Nitrokey/nitrokey-app#438.

  <releases>
    <release version="1.3.2" date="2018-08-21" />
    <release version="1.4.0" date="2019-06-25" />
  </releases>

I think this is semantically correct, but appstream-util validate does not like it :)

[hughsie]

It's quite unusual to merge branches together, no? I think the appstream-util check is useful, and has caught out bugs like not incrementing the year in the past.