Build healthcheck against gerrit stable-3.6

Bug: Issue 15831
Change-Id: If17070ba5c003e24638825377185387799becf71
diff --git a/jenkins/gerrit-bazel-plugin-healthcheck.yaml b/jenkins/gerrit-bazel-plugin-healthcheck.yaml
index 696a5ef..722726e 100644
--- a/jenkins/gerrit-bazel-plugin-healthcheck.yaml
+++ b/jenkins/gerrit-bazel-plugin-healthcheck.yaml
@@ -10,4 +10,5 @@
           branch:
             - master
           gerrit-branch:
+            - stable-3.6
             - stable-3.5