travis: add fc24 build

QubesOS/qubes-issues#1926

.travis.yml

@@ -7,5 +7,6 @@ before_script: sudo ln -s sid /usr/share/debootstrap/scripts/stretch
 script: ~/qubes-builder/scripts/travis-build
 env:
  - DISTS_VM=fc23 USE_QUBES_REPO_VERSION=3.2 USE_QUBES_REPO_TESTING=1
+ - DISTS_VM=fc24 USE_QUBES_REPO_VERSION=3.2 USE_QUBES_REPO_TESTING=1
  - DISTS_VM=jessie USE_QUBES_REPO_VERSION=3.2 USE_QUBES_REPO_TESTING=1
  - DISTS_VM=stretch USE_QUBES_REPO_VERSION=3.2 USE_QUBES_REPO_TESTING=1