Build reviewers plugin on stable-3.4 branch

Change-Id: I8bd1c62d861158db0380ef260d68609c9942a7d2
diff --git a/jenkins/gerrit-bazel-plugin-reviewers.yaml b/jenkins/gerrit-bazel-plugin-reviewers.yaml
index af51f8c..61d474d 100644
--- a/jenkins/gerrit-bazel-plugin-reviewers.yaml
+++ b/jenkins/gerrit-bazel-plugin-reviewers.yaml
@@ -5,6 +5,7 @@
           branch:
             - master
           gerrit-branch:
+            - stable-3.4
             - stable-3.3
       - 'plugin-{name}-bazel-{branch}':
           branch: