chore(package): update gulp to version 4.0.1

Closes #3

Author: greenkeeper[bot]

package.json

@@ -16,6 +16,6 @@
   },
   "devDependencies": {
     "chai": "^4.1.2",
-    "gulp": "^3.9.1"
+    "gulp": "^4.0.1"
   }
 }