travis: switch to dom0 Fedora 31

QubesOS/qubes-issues#5529

.travis.yml

@@ -16,7 +16,7 @@ script:
   - ./run-tests
   - ~/qubes-builder/scripts/travis-build
 env:
- - DIST_DOM0=fc29 USE_QUBES_REPO_VERSION=4.1 USE_QUBES_REPO_TESTING=1
+ - DIST_DOM0=fc31 USE_QUBES_REPO_VERSION=4.1 USE_QUBES_REPO_TESTING=1
 
 after_success:
   - codecov -F unittests