Build serviceuser plugin for Gerrit 3.1

Change-Id: I61cd576957ae6abeb933e9a8b9b33862a741fea0
diff --git a/jenkins/gerrit-bazel-plugin-serviceuser.yaml b/jenkins/gerrit-bazel-plugin-serviceuser.yaml
index 567de53..324d155 100644
--- a/jenkins/gerrit-bazel-plugin-serviceuser.yaml
+++ b/jenkins/gerrit-bazel-plugin-serviceuser.yaml
@@ -1,8 +1,13 @@
 - project:
     name: serviceuser
     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