Add slack-integration plugin to the check pipeline

Change-Id: Iea85073b484ea081ef913b5c36e441fdb6071fce
diff --git a/zuul.d/projects.yaml b/zuul.d/projects.yaml
index d6b66ec..37a513a 100644
--- a/zuul.d/projects.yaml
+++ b/zuul.d/projects.yaml
@@ -211,3 +211,9 @@
     check:
       jobs:
         - gerrit-plugin-build
+
+- project:
+    name: plugins/slack-integration
+    check:
+      jobs:
+        - gerrit-plugin-build