travis: add fc28 and buster

.travis.yml

@@ -7,8 +7,10 @@ script: ~/qubes-builder/scripts/travis-build
 env:
  - DISTS_VM=fc26 USE_QUBES_REPO_VERSION=4.0 USE_QUBES_REPO_TESTING=1
  - DISTS_VM=fc27 USE_QUBES_REPO_VERSION=4.0 USE_QUBES_REPO_TESTING=1
+ - DISTS_VM=fc28 USE_QUBES_REPO_VERSION=4.0 USE_QUBES_REPO_TESTING=1
  - DISTS_VM=jessie USE_QUBES_REPO_VERSION=4.0 USE_QUBES_REPO_TESTING=1
  - DISTS_VM=stretch USE_QUBES_REPO_VERSION=4.0 USE_QUBES_REPO_TESTING=1
+ - DISTS_VM=buster USE_QUBES_REPO_VERSION=4.0 USE_QUBES_REPO_TESTING=1
  - DISTS_VM=centos7 USE_QUBES_REPO_VERSION=4.0 USE_QUBES_REPO_TESTING=1
 
 jobs: