Build reviewers plugin for Gerrit v3.1

Change-Id: I1e8daa567c8de64aeb39bb341be59219037338ea
diff --git a/jenkins/gerrit-bazel-plugin-reviewers.yaml b/jenkins/gerrit-bazel-plugin-reviewers.yaml
index f98f7c0..3b7f90e 100644
--- a/jenkins/gerrit-bazel-plugin-reviewers.yaml
+++ b/jenkins/gerrit-bazel-plugin-reviewers.yaml
@@ -1,8 +1,13 @@
 - project:
     name: reviewers
     jobs:
-      - 'plugin-{name}-bazel-{branch}':
+      - 'plugin-{name}-bazel-{branch}-{gerrit-branch}':
           branch:
             - master
+          gerrit-branch:
+            - master
+            - stable-3.1
+      - 'plugin-{name}-bazel-{branch}':
+          branch:
             - stable-3.0
             - stable-2.16