Build stable-3.1 branch of events-log plugin

Change-Id: I3000b97636a5f5fb5e4ab8d59fe90595684d941f
diff --git a/jenkins/gerrit-bazel-plugin-events-log.yaml b/jenkins/gerrit-bazel-plugin-events-log.yaml
index 3b658b9..36a0296 100644
--- a/jenkins/gerrit-bazel-plugin-events-log.yaml
+++ b/jenkins/gerrit-bazel-plugin-events-log.yaml
@@ -4,5 +4,6 @@
       - 'plugin-{name}-bazel-{branch}':
           branch:
             - master
+            - stable-3.1
             - stable-3.0
             - stable-2.16