chore: Update package.json engines to Node.js 6+

Author: benurb

package.json

@@ -28,7 +28,7 @@
   },
   "homepage": "https://github.com/peerigon/extract-loader#readme",
   "engines": {
-    "node": ">= 4.0.0"
+    "node": ">= 6.0.0"
   },
   "devDependencies": {
     "babel-cli": "^6.26.0",