Bump gulp from 3.9.1 to 4.0.2

Bumps [gulp](https://github.com/gulpjs/gulp) from 3.9.1 to 4.0.2.
- [Release notes](https://github.com/gulpjs/gulp/releases)
- [Changelog](https://github.com/gulpjs/gulp/blob/master/CHANGELOG.md)
- [Commits](gulpjs/gulp@v3.9.1...v4.0.2)

Signed-off-by: dependabot-preview[bot] <[email protected]>

Author: dependabot-preview[bot]

package.json

@@ -13,7 +13,7 @@
   "devDependencies": {
     "eslint": "^6.8.0",
     "gh-pages": "^2.0.1",
-    "gulp": "^3.9.1",
+    "gulp": "^4.0.2",
     "gulp-browserify": "^0.5.1",
     "gulp-compile-handlebars": "^0.6.1",
     "gulp-concat": "^2.5.2",