travis: add centos7

.travis.yml

@@ -9,6 +9,7 @@ env:
  - DISTS_VM=fc27 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=centos7 USE_QUBES_REPO_VERSION=4.0 USE_QUBES_REPO_TESTING=1
 
 jobs:
   include: