travis: add fedora-31 & fedora-rawhide to the build images

The CentOS7 distro is quite old and the Ubuntu 18.04 distro
is already a year & half old. Adding a Fedora 31 image gives
us coverage of the newest stable distro release, and
fedora-rawhide gives us the cutting edge.
Reviewed-by: NFabiano Fidêncio <fidencio@redhat.com>
Signed-off-by: NDaniel P. Berrangé <berrange@redhat.com>

.travis.yml

@@ -31,6 +31,20 @@ matrix:
         - MAKE_ARGS="syntax-check distcheck"
       script:
         - make -C ci/ ci-build@$IMAGE CI_MAKE_ARGS="$MAKE_ARGS"
+    - services:
+        - docker
+      env:
+        - IMAGE="fedora-31"
+        - MAKE_ARGS="syntax-check distcheck"
+      script:
+        - make -C ci/ ci-build@$IMAGE CI_MAKE_ARGS="$MAKE_ARGS"
+    - services:
+        - docker
+      env:
+        - IMAGE="fedora-rawhide"
+        - MAKE_ARGS="syntax-check distcheck"
+      script:
+        - make -C ci/ ci-build@$IMAGE CI_MAKE_ARGS="$MAKE_ARGS"
     - services:
         - docker
       env: