chore(package): update gulp to version 4.0.1

Closes #65

Author: greenkeeper[bot]

package.json

@@ -34,7 +34,7 @@
 		"coveralls": "^3.0.2",
 		"eslint": "^5.8.0",
 		"eslint-config-cortys": "1.x",
-		"gulp": "3.9.1",
+		"gulp": "4.0.1",
 		"gulp-eslint": "^5.0.0",
 		"gulp-gh-pages": "0.5.4",
 		"gulp-istanbul": "^1.1.3",