Make sure benchmark runs in the production mode.

Author: arunoda

bench/package.json

@@ -2,7 +2,7 @@
   "name": "next-bench",
   "scripts": {
     "build": "next build",
-    "start": "npm run build && next start",
+    "start": "NODE_ENV=production npm run build && NODE_ENV=production next start",
     "bench:stateless": "ab -c1 -n3000 http://0.0.0.0:3000/stateless",
     "bench:stateless-big": "ab -c1 -n500 http://0.0.0.0:3000/stateless-big"
   }