Fixed #200: add "engines" field to package.json to indicate node version

Noted, this field can't avoid user to install it with wrong version of node,
so it just indicates what version of node we should use.

reference: http://www.marcusoft.net/2015/03/packagejson-and-engines-and-enginestrict.html

package.json

@@ -120,5 +120,9 @@
   },
   "pre-commit": [
     "lint"
-  ]
+  ],
+  "engineStrict": true,
+  "engines": {
+    "node": ">= 5.0.0"
+  }
 }