Build slack-integration plugin on its own stable-3.1 branch

Change-Id: I108b7af931a5e9df0e9f40e9b11c231c723a92de
diff --git a/jenkins/gerrit-bazel-plugin-slack-integration.yaml b/jenkins/gerrit-bazel-plugin-slack-integration.yaml
index 2f98aad..505ba14 100644
--- a/jenkins/gerrit-bazel-plugin-slack-integration.yaml
+++ b/jenkins/gerrit-bazel-plugin-slack-integration.yaml
@@ -7,8 +7,8 @@
           gerrit-branch:
             - master
             - stable-3.2
-            - stable-3.1
       - 'plugin-{name}-bazel-{branch}':
           branch:
+            - stable-3.1
             - stable-3.0
             - stable-2.16