fix(npm): npm run build as production

it prevent unnecessary power-assert module

Author: azu

package.json

@@ -22,7 +22,7 @@
     "test": "test"
   },
   "scripts": {
-    "build": "babel src --out-dir lib --source-maps",
+    "build": "NODE_ENV=production babel src --out-dir lib --source-maps",
     "watch": "babel src --out-dir lib --watch --source-maps",
     "prepublish": "npm run --if-present build",
     "test": "mocha"