Update avatars-external build job

- Build master and stable-3.1 against gerrit master
- Build stable-2.16 against gerrit stable-2.16
- Build stble-3.0 against gerrit stable-3.0

Change-Id: I81ca084cf884b207d34a4d19b8172b8915ba014c
diff --git a/jenkins/gerrit-bazel-plugin-avatars-external.yaml b/jenkins/gerrit-bazel-plugin-avatars-external.yaml
index dcbf739..57d10f8 100644
--- a/jenkins/gerrit-bazel-plugin-avatars-external.yaml
+++ b/jenkins/gerrit-bazel-plugin-avatars-external.yaml
@@ -6,5 +6,8 @@
             - master
           gerrit-branch:
             - master
+            - stable-3.1
+      - 'plugin-{name}-bazel-{branch}':
+          branch:
             - stable-3.0
             - stable-2.16