Build avatars-gravatar master against Gerrit stable-3.13

Change-Id: Id5a7b8c714427c2915d581ebd2e676205a32e6c9
diff --git a/jenkins/gerrit-bazel-plugin-avatars-gravatar.yaml b/jenkins/gerrit-bazel-plugin-avatars-gravatar.yaml
index 9393fbf..13d39e9 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-3.13
             - stable-3.12
             - stable-3.11
             - stable-3.10