Build slack-integration plugin for Gerrit v3.2

Change-Id: I0e906816e1e337331807ca030ec69c9605a63864
diff --git a/jenkins/gerrit-bazel-plugin-slack-integration.yaml b/jenkins/gerrit-bazel-plugin-slack-integration.yaml
index 80b676b..2f98aad 100644
--- a/jenkins/gerrit-bazel-plugin-slack-integration.yaml
+++ b/jenkins/gerrit-bazel-plugin-slack-integration.yaml
@@ -6,6 +6,7 @@
             - master
           gerrit-branch:
             - master
+            - stable-3.2
             - stable-3.1
       - 'plugin-{name}-bazel-{branch}':
           branch: