build(deps-dev): bump gulp from 3.9.1 to 4.0.2 in /site

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] <support@dependabot.com>

site/package.json

@@ -28,7 +28,7 @@
     "del": "^3.0.0",
     "eslint": "^4.3.0",
     "eslint-config-hexo": "^2.0.0",
-    "gulp": "^3.9.1",
+    "gulp": "^4.0.2",
     "gulp-cheerio": "^0.6.2",
     "gulp-clean-css": "^4.2.0",
     "gulp-if": "^2.0.0",