fix app.json (add YARN_PRODUCTION=false)

app.json

@@ -9,6 +9,7 @@
   "success_url": "/",
   "env": {
     "NODE_ENV": "production",
+    "YARN_PRODUCTION": false,
     "SECRET_TOKEN": {
       "description": "A secret key for verifying the integrity of signed cookies.",
       "generator": "secret"