Build avatars-gravatar plugin on stable-3.6

Change-Id: I4df6ac8b113127cfe365a13343f8eaace9b38dbd
diff --git a/jenkins/gerrit-bazel-plugin-avatars-gravatar.yaml b/jenkins/gerrit-bazel-plugin-avatars-gravatar.yaml
index 609dfce..bae8135 100644
--- a/jenkins/gerrit-bazel-plugin-avatars-gravatar.yaml
+++ b/jenkins/gerrit-bazel-plugin-avatars-gravatar.yaml
@@ -5,6 +5,7 @@
           branch:
             - master
           gerrit-branch:
+            - stable-3.6
             - stable-3.5
             - stable-3.4
             - stable-3.3