Build readonly plugin against gerrit 3.3

Change-Id: I3dec0e28d7d7ea9c5755b344cdeb834056f2c307
diff --git a/jenkins/gerrit-bazel-plugin-readonly.yaml b/jenkins/gerrit-bazel-plugin-readonly.yaml
index 4a2b1fb..d60cb9a 100644
--- a/jenkins/gerrit-bazel-plugin-readonly.yaml
+++ b/jenkins/gerrit-bazel-plugin-readonly.yaml
@@ -6,6 +6,7 @@
             - master
           gerrit-branch:
             - master
+            - stable-3.3
             - stable-3.2
       - 'plugin-{name}-bazel-{branch}':
           branch: