Add stable-3.{3,4} heartbeat master tree builds

Change-Id: I21e77de162e9ed549e3deabfd31e7673593a5822
diff --git a/jenkins/gerrit-bazel-plugin-heartbeat.yaml b/jenkins/gerrit-bazel-plugin-heartbeat.yaml
index 1bb6ff9..daed4ed 100644
--- a/jenkins/gerrit-bazel-plugin-heartbeat.yaml
+++ b/jenkins/gerrit-bazel-plugin-heartbeat.yaml
@@ -6,6 +6,8 @@
             - master
           gerrit-branch:
             - master
+            - stable-3.4
+            - stable-3.3
             - stable-3.2
       - 'plugin-{name}-bazel-{branch}':
           branch: