Update gulp requirement from ^3.9.1 to ^4.0.1

Updates the requirements on [gulp](https://github.com/gulpjs/gulp) to permit the latest version.
- [Release notes](https://github.com/gulpjs/gulp/releases)
- [Changelog](https://github.com/gulpjs/gulp/blob/master/CHANGELOG.md)
- [Commits](gulpjs/gulp@v3.9.1...v4.0.1)

Signed-off-by: dependabot[bot] <support@dependabot.com>

package.json

@@ -15,7 +15,7 @@
   "devDependencies": {
     "brace-expansion": "^1.1.4",
     "del": "^4.1.1",
-    "gulp": "^3.9.1",
+    "gulp": "^4.0.1",
     "gulp-exist": "^3.0.0",
     "gulp-watch": "^5.0.1",
     "gulp-less": "^4.0.1",