fix warning

Author: Francesco Rodriguez

test/sass_rails_test.rb

@@ -17,7 +17,7 @@ class SassRailsTest < Sass::Rails::TestCase
   test 'css_compressor config item is not honored in development mode' do
     within_rails_app 'alternate_config_project' do
       runcmd "ruby script/rails runner 'p Rails.application.config.assets.css_compressor'", Dir.pwd, true, "Gemfile", {"RAILS_ENV" => "development"}
-      assert_output /nil/
+      assert_equal 'nil', $last_ouput.chomp
     end
   end
 
@@ -52,7 +52,7 @@ class SassRailsTest < Sass::Rails::TestCase
   test 'sass not defines compressor in development mode' do
     within_rails_app 'scss_project' do
       runcmd "ruby script/rails runner 'p Rails.application.config.assets.css_compressor'", Dir.pwd, true, 'Gemfile', {'RAILS_ENV' => 'development'}
-      assert_output /nil/
+      assert_equal 'nil', $last_ouput.chomp
     end
   end