Update gulp-mocha requirement from ^2.0.0 to ^6.0.0
Update gulp-mocha requirement from ^2.0.0 to ^6.0.0

Updates the requirements on [gulp-mocha](https://github.com/sindresorhus/gulp-mocha) to permit the latest version.
- [Release notes](https://github.com/sindresorhus/gulp-mocha/releases)
- [Commits](https://github.com/sindresorhus/gulp-mocha/commits/v6.0.0)

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