Solution: add new tokens to .travis.yml and change ci_deploy.sh
script to use Github APIs to create a temporary branch at the tag,
and the OBS APIs to trigger a source service run in the stable and
draft release projects:
network:messaging:zeromq:release-stable
network:messaging:zeromq:release-draft

The branch hack is unfortunately necessary as it is not possible to
modify OBS sources with the token APIs, and it is also not possible
to automatically fetch the latest tag in the service files.
The temporary branch is immediately deleted.