libsass: 3.5.4 -> 3.5.5

[florianjacob]

Motivation for this change

3.5.5 reverts the css import breaking behaviour from 3.5.3:
https://github.com/sass/libsass/releases

It also contains the included security patch, which could be therefore removed:
https://github.com/sass/libsass/commits/3.5.5

Not totally sure on backporting - this update breaks the current behaviour, but if you define the current behaviour as broken, for which there are reasons, it's more of an un-breaking than a breaking change.

Maintainers: @codyopel @offlinehacker

Things done

• Tested using sandboxing (nix.useSandbox on NixOS, or option sandbox in nix.conf on non-NixOS)
• Built on platform(s)
  • NixOS
  • macOS
  • other Linux distributions
• Tested via one or more NixOS test(s) if existing and applicable for the change (look inside nixos/tests)
• Tested compilation of all pkgs that depend on this change using nix-shell -p nox --run "nox-review wip"
• Tested execution of all binary files (usually in ./result/bin/)
• Determined the impact on package closure size (by running nix path-info -S before and after)
• Assured whether relevant documentation is up to date
• Fits CONTRIBUTING.md.

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[florianjacob]

@GrahamcOfBorg build libsass