Fixed an error caused by compiling :not selectors.

Fixes #1476 

From what I can see it is [this commit](https://github.com/sass/sass/commit/c717726c4307899bec87482673bfb7fb6fe1f509) which has introduced the bug. The fact that it now [returns an Array](https://github.com/sass/sass/blob/c717726c4307899bec87482673bfb7fb6fe1f509/lib/sass/selector/pseudo.rb#L93-L97) is not being handled in [this method](https://github.com/sass/sass/blob/stable/lib/sass/selector/abstract_sequence.rb#L96) - it is expecting an instance of the `Sass::Selector::Pseudo` class, but is instead now given an `Array` containing the instance.

This PR reverts the changes made in the `with_selector` method, and adds the introduced `:not` logic into a new method which is called in the relevant place.

Tests pass - let me know if this is any good and if I need to update anything, including the changelog.