将 sass-rails 版本更改为 4.0.3，其余保持原样.必要时删除 Gemfile.lock，然后:

Change sass-rails version to 4.0.3 and leave the rest as it is. Remove Gemfile.lock when necessary, and then:

$ bundle update
$ bundle install

最后，不要忘记重启你的rails服务器！

Finally, don't forget to restart your rails server!