Deal with Ruby Sass deprecation

[zanedb]

Ruby Sass is deprecated and will be unmaintained as of 26 March 2019.

• If you use Sass as a command-line tool, we recommend using Dart Sass, the new
  primary implementation: https://sass-lang.com/install
• If you use Sass as a plug-in for a Ruby web framework, we recommend using the
  sassc gem: https://github.com/sass/sassc-ruby#readme
• For more details, please refer to the Sass blog:
  http://sass.logdown.com/posts/7081811