travis: add build for Debian 9

Debian 9 ships the oldest versions of some of our dependent
packages so can highlight bugs not seen elsewhere. CentOS 7,
despite being quite old, has rebased some packages to much
newer versions.
Reviewed-by: NPavel Hrdina <phrdina@redhat.com>
Reviewed-by: NDaniel Henrique Barboza <danielhb413@gmail.com>
Signed-off-by: NDaniel P. Berrangé <berrange@redhat.com>

.travis.yml

@@ -32,6 +32,13 @@ matrix:
         - MAKE_ARGS="syntax-check distcheck"
       script:
         - make -C ci/ ci-build@$IMAGE CI_MAKE_ARGS="$MAKE_ARGS"
+    - services:
+        - docker
+      env:
+        - IMAGE="debian-9"
+        - MAKE_ARGS="syntax-check distcheck"
+      script:
+        - make -C ci/ ci-build@$IMAGE CI_MAKE_ARGS="$MAKE_ARGS"
     - services:
         - docker
       env: