Build heartbeat against gerrit stable-3.6

Bug: Issue 15831
Change-Id: I5389181a0d793cfc378b60bc24760112b0cfc044
diff --git a/jenkins/gerrit-bazel-plugin-heartbeat.yaml b/jenkins/gerrit-bazel-plugin-heartbeat.yaml
index aa5788c..367de66 100644
--- a/jenkins/gerrit-bazel-plugin-heartbeat.yaml
+++ b/jenkins/gerrit-bazel-plugin-heartbeat.yaml
@@ -6,6 +6,7 @@
             - master
           gerrit-branch:
             - master
+            - stable-3.6
             - stable-3.5
             - stable-3.4
             - stable-3.3