Upgrade gulp-clean-css to match clean-css version 5.*

[alexeych]

Please, consider upgrading gulp-clean-css to 5.* version to match with the latest version of clean-css.

[thomasfrobieter]

The old version causes a critical issue in meantime, this thing really needs updated deps.

[sevikerr]

@scniro, could you tell us, do you want to maintain this package?
There are no updates for 2 years.

[kerryj89]

Here's a fork that appears better maintained https://github.com/aptuitiv/gulp-clean-css