bump min node version

Author: marcelduran

package.json

@@ -25,7 +25,7 @@
     "test": "./node_modules/mocha/bin/mocha -R spec test/*-test.js"
   },
   "engines": {
-    "node": ">=4.9.1  "
+    "node": ">=6.4.0"
   },
   "dependencies": {
     "commander": "^5.1.0",