Add node.js version requirement to package.json

Author: Minggang Wang

package.json

@@ -42,5 +42,8 @@
     "ref-array": "^1.2.0",
     "walk": "^2.3.9",
     "nan": "^2.8.0"
+  },
+  "engines": {
+    "node": ">= 6.10.0"
   }
 }