Build its-base on stable-3.1

Change-Id: I6c437ead6765d108b0450034999a7626721e9e43
diff --git a/jenkins/gerrit-bazel-plugin-its-base.yaml b/jenkins/gerrit-bazel-plugin-its-base.yaml
index cc34000..4cc1964 100644
--- a/jenkins/gerrit-bazel-plugin-its-base.yaml
+++ b/jenkins/gerrit-bazel-plugin-its-base.yaml
@@ -4,6 +4,7 @@
       - 'plugin-{name}-bazel-{branch}':
           branch:
             - master
+            - stable-3.1
             - stable-3.0
             - stable-2.16
           targets: 'plugins/its-base:its-base plugins/its-base:its_base_tests'