Update gulp requirement from ^3.9.1 to ^4.0.0
Update gulp requirement from ^3.9.1 to ^4.0.0

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](https://github.com/gulpjs/gulp/commits/v4.0.0)

Signed-off-by: dependabot[bot] <5bdcd3c0d4d24ae3e71b3b452a024c6324c7e4bb@dependabot.com>
hexojs/hexo-html-minifier