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

bundle install したら、見慣れないメッセージが。

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

sasscの方にしろって。

vi Gemfile
>>>
#gem 'sass-rails', '~> 5.0'
gem 'sassc-rails'
<<<

vi Gemfile.lock
>>> sass辺りを削除
-    sass-rails (5.0.7)
-      railties (>= 4.0.0, < 6)
-      sass (~> 3.1)
-      sprockets (>= 2.8, < 4.0)
-      sprockets-rails (>= 2.0, < 4.0)
-      tilt (>= 1.1, < 3)
<<<

gem uninstall sass-rails
bundle install