Replace Ruby Sass with some alternative(s)

bundle install says as follows:

[...]
Installing sass 3.6.0
[...]
Bundle complete! 13 Gemfile dependencies, 57 gems now installed.
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

Also see the official web site at https://sass-lang.com/ruby-sass for more information.

Ruby Sass was the original implementation of Sass, but it's since been deprecated. It's currently in a sunset maintenance period, and it will be officially unmaintained as of 26 March 2019. Ruby Sass users should migrate to another implementation.