Build stable-3.9 for its-phabricator

Change-Id: I5fcc3eafdc97fc59278e3013f919bc3045adc98b
diff --git a/jenkins/gerrit-bazel-plugin-its-phabricator.yaml b/jenkins/gerrit-bazel-plugin-its-phabricator.yaml
index 00560b7..1138d6d 100644
--- a/jenkins/gerrit-bazel-plugin-its-phabricator.yaml
+++ b/jenkins/gerrit-bazel-plugin-its-phabricator.yaml
@@ -4,3 +4,4 @@
       - 'plugin-its-{name}-bazel-{branch}':
           branch:
             - master
+            - stable-3.9