Bump jekyll-sass-converter to v2.1.0

[hubot]

Release: https://github.com/jekyll/jekyll-sass-converter/releases/tag/v2.1.0 Diff: https://github.com/jekyll/jekyll-sass-converter/compare/v2.1.0...v2.1.0

[parkr]

Requires Jekyll 4.x (#791) or it requires Jekyll 3 to loosen it's requirements and allow jekyll-sass-converter 2.x. The project has done that in the past, so it's not too much to ask as long as it's strictly backwards compatible.