Build task plugin on its own stable-3.1 branch

Change-Id: I73332f5156f5d15ccd14bf26bef05490209ba518
diff --git a/jenkins/gerrit-bazel-plugin-task.yaml b/jenkins/gerrit-bazel-plugin-task.yaml
index 9c4665a..729ba25 100644
--- a/jenkins/gerrit-bazel-plugin-task.yaml
+++ b/jenkins/gerrit-bazel-plugin-task.yaml
@@ -6,10 +6,10 @@
             - master
           gerrit-branch:
             - master
-            - stable-3.1
             - stable-3.2
       - 'plugin-{name}-bazel-{branch}':
           branch:
             - master
+            - stable-3.1
             - stable-3.0
             - stable-2.16