Bug 1286075: set tier for eslint; r=Mossop

MozReview-Commit-ID: JBvvrAHj3Ry

--HG--
extra : rebase_source : a6dcd680fad6a8b982da3732d7c548b10e650458

taskcluster/ci/legacy/tasks/tests/mozlint-eslint.yml

@@ -41,6 +41,7 @@ task:
       machine:
         platform: lint
       symbol: ES
+      tier: 1
     treeherderEnv:
       - production
       - staging