[PATCH] update list_releases() release list

[Stefano Zacchiroli]

Here are my proposed changes for #758027 ("debian_support's
list_releases() has outdated list of releases", reported by Stuart.  As
discussed here, my changes implement the easiest option (a), i.e. just
updating the release list.

If there are no objections with my changes, and considering no one else
has proposed other pending changes before the freeze, I'd like to go
ahead with an upload to unstable before next week. (Of course we can
still squeeze in other changes with future uploads, but in the mean time
I'd like to get the current state of the master branch into testing.)

Feedback welcome!
Cheers.