ci: Use Ubuntu Bionic for websocket-autobahn job
Autobahn testsuite is based on Python 2. See https://github.com/crossbario/autobahn-testsuite/issues/109.
Temporarily use an Ubuntu LTS distro still having Python 2, this will make the job work for some years.
Hoping in the future to either:
- update Autobahn testsuite to Python 3;
- finding another testsuite for websockets.
In the series there's another small patch to fix a compiler error for G++ version in Bionic.
This supersede !155 (closed)