Build slack-integration plugin on stable-3.4 branch

Change-Id: Ibe0a10e2b851417919c7aac58a0fe09ffac6124b
diff --git a/jenkins/gerrit-bazel-plugin-slack-integration.yaml b/jenkins/gerrit-bazel-plugin-slack-integration.yaml
index c2acb35..3a071b5 100644
--- a/jenkins/gerrit-bazel-plugin-slack-integration.yaml
+++ b/jenkins/gerrit-bazel-plugin-slack-integration.yaml
@@ -1,9 +1,14 @@
 - project:
     name: slack-integration
     jobs:
-      - 'plugin-{name}-bazel-{branch}':
+      - 'plugin-{name}-bazel-{branch}-{gerrit-branch}':
           branch:
             - master
+          gerrit-branch:
+            - master
+            - stable-3.4
+      - 'plugin-{name}-bazel-{branch}':
+          branch:
             - stable-3.3
             - stable-3.2
             - stable-3.1