Build changemessage plugin on its own stable-3.1 branch

Change-Id: I0ffc2e951bb9715f3d6296c4f214a8daa2ecd7d0
diff --git a/jenkins/gerrit-bazel-plugin-changemessage.yaml b/jenkins/gerrit-bazel-plugin-changemessage.yaml
index 600dc8b..d36d0a3 100644
--- a/jenkins/gerrit-bazel-plugin-changemessage.yaml
+++ b/jenkins/gerrit-bazel-plugin-changemessage.yaml
@@ -7,9 +7,9 @@
           gerrit-branch:
             - master
             - stable-3.2
-            - stable-3.1
       - 'plugin-{name}-bazel-{branch}':
           branch:
             - master
+            - stable-3.1
             - stable-3.0
             - stable-2.16