Deal with new warning from Ruby Sass

[tarkatronic]

After processing the updates for #106, saas gave the following warning:

Post-install message from sass:

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

It appears there may be a suitable replacement recommended in the above message, so actions should be taken so as to get rid of this warning.