build(package.json): Raise minimum Node version to LTS argon version

Changed `engine` semvar to match last version of Node-4.x. Its EOL is scheduled for 2018-04-30.

package.json

@@ -83,7 +83,7 @@
     "time-grunt": "~1.4.0"
   },
   "engines": {
-    "node": ">= 4.0.0",
+    "node": ">= 4.8.7",
     "npm": ">= 2.15.11"
   }
 }