travis: add fc26 build

.travis.yml

@@ -7,6 +7,7 @@ script: ~/qubes-builder/scripts/travis-build
 env:
  - DISTS_VM=fc24 USE_QUBES_REPO_VERSION=4.0 USE_QUBES_REPO_TESTING=1
  - DISTS_VM=fc25 USE_QUBES_REPO_VERSION=4.0 USE_QUBES_REPO_TESTING=1
+ - DISTS_VM=fc26 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