Build simple-submit-rules plugin on stable-3.1

Change-Id: I66215edc3120611e5e959ca02686938ab2d8f26f
diff --git a/jenkins/gerrit-bazel-plugin-simple-submit-rules.yaml b/jenkins/gerrit-bazel-plugin-simple-submit-rules.yaml
index 6580ece..a839780 100644
--- a/jenkins/gerrit-bazel-plugin-simple-submit-rules.yaml
+++ b/jenkins/gerrit-bazel-plugin-simple-submit-rules.yaml
@@ -4,5 +4,6 @@
       - 'plugin-{name}-bazel-{branch}':
           branch:
             - master
+            - stable-3.1
             - stable-3.0
             - stable-2.16