Add sass-rails back to the default Gemfile

It was removed by mistake at 877a411d

railties/lib/rails/generators/app_base.rb

@@ -281,6 +281,8 @@ def assets_gemfile_entry
         return [] if options[:skip_sprockets]
 
         gems = []
+        gems << GemfileEntry.version('sass-rails', '~> 5.0',
+                                     'Use SCSS for stylesheets')
 
         gems << GemfileEntry.version('uglifier',
                                    '>= 1.3.0',

railties/test/generators/app_generator_test.rb

@@ -361,6 +361,13 @@ def test_generator_if_skip_action_mailer_is_given
     end
   end
 
+  def test_generator_has_assets_gems
+    run_generator
+
+    assert_gem 'sass-rails'
+    assert_gem 'uglifier'
+  end
+
   def test_generator_if_skip_sprockets_is_given
     run_generator [destination_root, "--skip-sprockets"]
     assert_no_file "config/initializers/assets.rb"