travis: Remove Ubuntu Cosmic, add Eoan and Focal

Remove the EOL'd Ubuntu 18.10 "Cosmic Cattlefish" release, add
Ubuntu 19.10 "Eoan Ermine" and Ubuntu 20.04 LTS "Focal Fossa".

Reflect the change of naming scheme in the quassel-build-env
repository, where branches (and docker tags) are now prefixed
with distro name to prepare for adding additional distros to CI.