Move checks plugin out of the core project list

We started a new section for non-core projects, move the checks
plugin there.

Change-Id: I5b9b7887f0d2bbac3f33d667ee4f764e7b5a79b5

zuul/main.yaml

diff --git a/zuul/main.yaml b/zuul/main.yaml
index 93474a7..caf3f4d 100644
--- a/zuul/main.yaml
+++ b/zuul/main.yaml
@@ -17,7 +17,6 @@
           # All of the core projects, alphabetically sorted.
           - gerrit
           - jgit
-          - plugins/checks
           - plugins/codemirror-editor
           - plugins/commit-message-length-validator
           - plugins/delete-project
@@ -31,4 +30,5 @@
           - plugins/webhooks
           - polymer-bridges
           # All of the rest of the projects, alphabetically sorted.
+          - plugins/checks
           - plugins/code-owners