options version 3x

As far as i understand the option should be changed according https://github.com/jakubpawlowicz/clean-css#how-to-upgrade-clean-css-from-2x-to-3x

Both `advanced` and `rebase` are set to false by default now.
Btw. can't we just remove these conditions at all? Obviously  undefined  is already  false  by default.