Upgrade for compatibility with stylelint 9.x.x

[kaelig]

See https://github.com/stylelint/stylelint/blob/master/CHANGELOG.md#900

9.0.0

• Removed: Node.JS 4.x support. Node.js 6.x or greater is now required (#3075).
• Added: (experimental) support for SASS syntax (#2503).
• Added: allow processors to handle PostCSS errors (#3063).
• Added: --max-warnings CLI flag (#2942).
• Added: selector-combinator-*list rules (#3088).
• Added: selector-pseudo-element-*list rules (#3104).
• Added: ignore: ["first-nested"] to custom-property-empty-line-before (#3104).
• Added: ignore: ["first-nested"] to declaration-empty-line-before (#3103).
• Added: ignoreProperties: [] to property-no-vendor-prefix (#3089).
• Fixed: font-family-name-quotes unicode range increased (#2974).
• Fixed: selector-max-id in nested at-statements (#3113).

[kaelig]

Fixed in v5.