Update jekyll-sass-converter to the latest RC

To see a detailed history of the changes, please visit https://github.com/jekyll/jekyll-sass-converter/compare/v1.0.0.rc3...v1.0.0.rc4

Notable change is that you can now set the style of the outputted Sass.
Ref: https://github.com/jekyll/jekyll-sass-converter/issues/3