Specify node engine version.

Specify the newest version of node at the time to prevent
backward compatibility issues.

Author: coderkevin

examples/simple/package.json

@@ -44,5 +44,6 @@
     "webpack": "^1.12.9",
     "webpack-dev-middleware": "^1.4.0",
     "webpack-hot-middleware": "^2.6.0"
-  }
+  },
+	"engine": "node >= 5.7.1"
 }