travis-ci: remove Ubuntu Disco, add Focal

Ubuntu Disco is EOL and fails in CI on apt repositories updating with
'does not have a Release file' errors.

Author: LeonidVas

.travis.yml

@@ -104,10 +104,10 @@ jobs:
       env: OS=ubuntu DIST=xenial
     - <<: *packpack
       env: OS=ubuntu DIST=bionic
-    - <<: *packpack
-      env: OS=ubuntu DIST=disco
     - <<: *packpack
       env: OS=ubuntu DIST=eoan
+    - <<: *packpack
+      env: OS=ubuntu DIST=focal
     - <<: *packpack
       env: OS=debian DIST=jessie
     - <<: *packpack