Build lfs plugin on its new stable-3.1 branch

As the plugin builds for 3.1 only with that adaptation in.

Change-Id: Ife4304f7e875db1fcb9cc98e4027eb41f38155b0
diff --git a/jenkins/gerrit-bazel-plugin-lfs.yaml b/jenkins/gerrit-bazel-plugin-lfs.yaml
index 4fba980..d74c6be 100644
--- a/jenkins/gerrit-bazel-plugin-lfs.yaml
+++ b/jenkins/gerrit-bazel-plugin-lfs.yaml
@@ -4,5 +4,6 @@
       - 'plugin-{name}-bazel-{branch}':
           branch:
             - master
+            - stable-3.1
             - stable-3.0
             - stable-2.16