Gem install json-v1.7.3提供“请更新 PATH 以包含构建工具”

当我跑步的时候:

rails new blog

我得到了:

Installing json (1.7.3)
Gem::InstallError: The 'json' native gem requires installed build tools.


Please update your PATH to include build tools or download the DevKit
from 'http://rubyinstaller.org/downloads' and follow the instructions
at 'http://github.com/oneclick/rubyinstaller/wiki/Development-Kit'
An error occurred while installing json (1.7.3), and Bundler cannot continue.
Make sure that `gem install json -v '1.7.3'` succeeds before bundling.

当我跑步的时候:

gem install json -v 1.7.3

我得到了:

Please update your PATH to include build tools or download the DevKit
from 'http://rubyinstaller.org/downloads' and follow the instructions
at 'http://github.com/oneclick/rubyinstaller/wiki/Development-Kit'

我用的是 Windows 8。我的路径上确实有 c:\ruby193\bin。我该如何纠正这些错误呢？