travis: switch to dom0 Fedora 31

QubesOS/qubes-issues#5529

.travis.yml

@@ -11,7 +11,7 @@ script:
  - ~/qubes-builder/scripts/travis-build
 env:
  - DIST_DOM0=fc25 USE_QUBES_REPO_VERSION=4.0 USE_QUBES_REPO_TESTING=1
- - 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
  - DISTS_VM=fc30 USE_QUBES_REPO_VERSION=4.1 USE_QUBES_REPO_TESTING=1
  - DISTS_VM=fc31 USE_QUBES_REPO_VERSION=4.1 USE_QUBES_REPO_TESTING=1
  - DISTS_VM=centos7 USE_QUBES_REPO_VERSION=4.1 USE_QUBES_REPO_TESTING=1