The sass gem has been deprecated and has reached EOL, this replaces i…

…t with the recommended sassc gem (#379)

rack-mini-profiler.gemspec

@@ -32,7 +32,7 @@ Gem::Specification.new do |s|
   s.add_development_dependency 'dalli'
   s.add_development_dependency 'rspec', '~> 3.6.0'
   s.add_development_dependency 'redis'
-  s.add_development_dependency 'sass'
+  s.add_development_dependency 'sassc'
   s.add_development_dependency 'flamegraph'
   s.add_development_dependency 'rubocop'