Build readonly plugin on stable-3.1

Change-Id: I5f04b8f5b68e54b528ff83a825bc167222ff0bb8
diff --git a/jenkins/gerrit-bazel-plugin-readonly.yaml b/jenkins/gerrit-bazel-plugin-readonly.yaml
index 6f64f97..abeaa58 100644
--- a/jenkins/gerrit-bazel-plugin-readonly.yaml
+++ b/jenkins/gerrit-bazel-plugin-readonly.yaml
@@ -4,5 +4,6 @@
       - 'plugin-{name}-bazel-{branch}':
           branch:
             - master
+            - stable-3.1
             - stable-3.0
             - stable-2.16