Build ref-protection plugin on stable-3.0/3.1

Change-Id: I4a369a9a149ab5034afd6acfd812c974f769efdd
diff --git a/jenkins/gerrit-bazel-plugin-ref-protection.yaml b/jenkins/gerrit-bazel-plugin-ref-protection.yaml
index 9d0b298..9741592 100644
--- a/jenkins/gerrit-bazel-plugin-ref-protection.yaml
+++ b/jenkins/gerrit-bazel-plugin-ref-protection.yaml
@@ -1,6 +1,13 @@
 - project:
     name: ref-protection
     jobs:
+      - 'plugin-{name}-bazel-{branch}-{gerrit-branch}':
+          branch:
+            - master
+          gerrit-branch:
+            - master
+            - stable-3.1
+            - stable-3.0
       - 'plugin-{name}-bazel-{branch}':
           branch:
             - master