chore(package): update gulp to version 4.0.1

Closes #21

Author: greenkeeper[bot]

package.json

@@ -35,7 +35,7 @@
   },
   "devDependencies": {
     "chai": "^4.0.1",
-    "gulp": "^3.9.1",
+    "gulp": "^4.0.1",
     "mocha": "^4.0.0",
     "stream-assert": "^2.0.3"
   }