chore(package): update gulp to version 4.0.2

Closes #29

Author: greenkeeper[bot]

package.json

@@ -26,7 +26,7 @@
     "eslint": "^4.18.0",
     "eslint-config-prettier": "^2.9.0",
     "eslint-plugin-prettier": "^2.6.0",
-    "gulp": "^3.9.1",
+    "gulp": "^4.0.2",
     "gulp-codacy": "^1.0.0",
     "gulp-coveralls": "^0.1.4",
     "gulp-debug": "^3.2.0",