travis-ci: remove Ubuntu Disco

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

Author: artembo

Diff for: .travis.yml

@@ -15,7 +15,6 @@ env:
       - OS=ubuntu DIST=trusty
       - OS=ubuntu DIST=xenial
       - OS=ubuntu DIST=bionic
-      - OS=ubuntu DIST=disco
       - OS=debian DIST=jessie
       - OS=debian DIST=stretch
       - OS=debian DIST=buster