chore(package): update gulp to version 4.0.1

Closes #111

Author: greenkeeper[bot]

package.json

@@ -64,7 +64,7 @@
     "babel-preset-es2017": "^6.24.1",
     "babel-require": "^1.0.1",
     "del": "^3.0.0",
-    "gulp": "^3.9.1",
+    "gulp": "^4.0.1",
     "gulp-bump": "^3.1.0",
     "gulp-filter": "^5.1.0",
     "gulp-git": "^2.5.1",