docs: Use Debian stretch packages for Ubuntu 18.04
docs: Use Debian stretch packages for Ubuntu 18.04

kingosticks/mopidy,adamcik/mopidy,mopidy/mopidy,jodal/mopidy,jodal/mopidy,kingosticks/mopidy,jcass77/mopidy,kingosticks/mopidy,jodal/mopidy,jcass77/mopidy,jcass77/mopidy,adamcik/mopidy,mopidy/mopidy,mopidy/mopidy,adamcik/mopidy