Remove sass dependency

This mimics how we've done it in Bourbon:
thoughtbot/bourbon@9bedac0

Author: tysongach

lib/neat.rb

@@ -1,12 +1,15 @@
-require "sass"
 require "neat/generator"
 
 module Neat
-  if defined?(Rails::Engine)
+  if defined?(Rails) && defined?(Rails::Engine)
     class Engine < ::Rails::Engine
       config.assets.paths << File.expand_path("../core", __dir__)
     end
   else
-    Sass.load_paths << File.expand_path("../core", __dir__)
+    begin
+      require "sass"
+      Sass.load_paths << File.expand_path("../core", __dir__)
+    rescue LoadError
+    end
   end
 end

neat.gemspec

@@ -7,7 +7,6 @@ Gem::Specification.new do |s|
   s.add_development_dependency "rake"
   s.add_development_dependency "rspec"
   s.add_development_dependency "scss_lint", "~> 0.44"
-  s.add_runtime_dependency "sass", "~> 3.4"
   s.add_runtime_dependency "thor", "~> 0.19"
   s.authors = [
     "Joel Oliveira",