Ubuntu’s apt started refusing to load package files from unauthenticated
repositories, but the package for which we did that (xcb-xrm) is available in
newer versions of Ubuntu, so I just removed that part altogether.
Apparently this has been broken since April, and nobody noticed :-/
The resulting packages are pushed to Debian repositories hosted on
bintray.com.
This is the first step to move away from our custom buildbot setup (see
https://i3wm.org/docs/buildbot.html for details on that) towards
infrastructure which is more standard (travis) and in the open.
