travis: add CentOS 7 build

.travis.yml

@@ -14,6 +14,7 @@ env:
  - DIST_DOM0=fc29 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
 
 # don't build tags which are meant for code signing only
 branches: