Update 'engines' field in package.json to note that we require Node.js v4

Author: marco-c

package.json

@@ -57,6 +57,6 @@
     "which": "^1.2.11"
   },
   "engines": {
-    "node": ">= v0.10.0"
+    "node": ">= 4"
   }
 }