安装Sass时出错(Ruby 2.5.0.1,MSYS2 20161025.0.0)

choco install ruby
choco install msys2

看似他们已正确安装. ruby -v导致：

ruby 2.5.0p0 (2017-12-25 revision 61468) [x64-mingw32]

当我尝试运行gem install sass时收到以下错误消息：

ERROR:  Error installing sass:
        The last version of ffi (< 2,>= 0.5.0) to support your Ruby & RubyGems was 1.9.18. Try installing it with `gem install ffi -v 1.9.18` and then running the current command again
        ffi requires Ruby version < 2.5,>= 2.0. The current ruby version is 2.5.0.

好的,所以我试图运行gem install ffi -v 1.9.18.

我收到此错误消息：

ERROR:  Error installing ffi:
        The last version of ffi (= 1.9.18) to support your Ruby & RubyGems was 1.9.18. Try installing it with `gem install ffi -v 1.9.18`
        ffi requires Ruby version < 2.5,>= 2.0. The current ruby version is 2.5.0.

我尝试使用适当的安装程序重新安装Ruby,重新启动计算机,但出现相同的错误.

如何让Sass安装？