travis: Update name for Ubuntu 18.04 image

The corresponding libvirt-jenkins-ci commit is f289e64a5fd9.
Signed-off-by: NAndrea Bolognani <abologna@redhat.com>

.travis.yml

@@ -21,7 +21,7 @@ matrix:
     - services:
         - docker
       env:
-        - IMAGE="ubuntu-18"
+        - IMAGE="ubuntu-1804"
         - MAKE_ARGS="syntax-check distcheck"
       script:
         - make -C ci/ ci-build@$IMAGE CI_MAKE_ARGS="$MAKE_ARGS"