Build project-group-structure plugin on its own stable-3.1 branch

Change-Id: I7dafd0ab405a5d8acb76b77945b41234e822d28f
diff --git a/jenkins/gerrit-bazel-plugin-project-group-structure.yaml b/jenkins/gerrit-bazel-plugin-project-group-structure.yaml
index c240093..acd6082 100644
--- a/jenkins/gerrit-bazel-plugin-project-group-structure.yaml
+++ b/jenkins/gerrit-bazel-plugin-project-group-structure.yaml
@@ -6,9 +6,9 @@
             - master
           gerrit-branch:
             - master
-            - stable-3.1
             - stable-3.2
       - 'plugin-{name}-bazel-{branch}':
           branch:
+            - stable-3.1
             - stable-3.0
             - stable-2.16