Pin sassc to 2.1.0

sass/sassc-ruby#146 (comment)

Gemfile

@@ -40,6 +40,7 @@ gem 'prawn'
 gem 'prawn-table'
 gem 'rails-i18n', '~> 6.0.0'
 gem 'rubocop'
+gem 'sassc', '2.1.0' # https://github.com/sass/sassc-ruby/issues/146#issuecomment-528752874
 
 group :development do
   # Access an interactive console on exception pages or by calling 'console' anywhere in the code.