Build server-config plugin for stable-3.1

Change-Id: I96501d4115f8151de74ec8ac6eb22691e0bd2560
diff --git a/jenkins/gerrit-bazel-plugin-server-config.yaml b/jenkins/gerrit-bazel-plugin-server-config.yaml
index 9be7bae..a6400c2 100644
--- a/jenkins/gerrit-bazel-plugin-server-config.yaml
+++ b/jenkins/gerrit-bazel-plugin-server-config.yaml
@@ -4,3 +4,4 @@
       - 'plugin-{name}-bazel-{branch}':
           branch:
             - master
+            - stable-3.1