Build avatars-gravatar plugin on stable-3.4

Change-Id: If215d9b34cbf9c142d4fce448d7828302db98096
diff --git a/jenkins/gerrit-bazel-plugin-avatars-gravatar.yaml b/jenkins/gerrit-bazel-plugin-avatars-gravatar.yaml
index ae81488..eb584b4 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-3.4
             - stable-3.3
             - stable-3.2
             - stable-3.1