chore(package): update gulp to version 4.0.1

Closes #21

Author: greenkeeper[bot]

package.json

@@ -70,7 +70,7 @@
     "cssnano": "^4.1.10",
     "del": "^3.0.0",
     "fancy-log": "^1.3.3",
-    "gulp": "^3.9.1",
+    "gulp": "^4.0.1",
     "gulp-bump": "^3.1.1",
     "gulp-conventional-changelog": "^2.0.10",
     "gulp-coveralls": "^0.1.4",