Build automerger plugin on stable-3.4 branch

Change-Id: I36830aabddcbbb32cf540ac1ce1f0c8619590460
diff --git a/jenkins/gerrit-bazel-plugin-automerger.yaml b/jenkins/gerrit-bazel-plugin-automerger.yaml
index c21ab9f..c4a7257 100644
--- a/jenkins/gerrit-bazel-plugin-automerger.yaml
+++ b/jenkins/gerrit-bazel-plugin-automerger.yaml
@@ -6,6 +6,7 @@
             - master
           gerrit-branch:
             - master
+            - stable-3.4
             - stable-3.3
       - 'plugin-{name}-bazel-{branch}':
           branch: