travis-ci: change ubuntu disco CI with focal

Ubuntu disco has reached EOL in january. It's time to
upgrade.

Closes #249

Author: rosik

.travis.yml

@@ -31,7 +31,7 @@ env:
       - OS=ubuntu DIST=trusty
       - OS=ubuntu DIST=xenial
       - OS=ubuntu DIST=bionic
-      - OS=ubuntu DIST=disco
+      - OS=ubuntu DIST=focal
       - OS=debian DIST=jessie
       - OS=debian DIST=stretch
       - OS=debian DIST=buster