Build slack-integration plugin for stable-3.1

Change-Id: I053f0ca043c54df5b2d22244a290cc7567f5eb6a
diff --git a/jenkins/gerrit-bazel-plugin-slack-integration.yaml b/jenkins/gerrit-bazel-plugin-slack-integration.yaml
index 9e19c47..80b676b 100644
--- a/jenkins/gerrit-bazel-plugin-slack-integration.yaml
+++ b/jenkins/gerrit-bazel-plugin-slack-integration.yaml
@@ -1,8 +1,13 @@
 - project:
     name: slack-integration
     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