错误：执行gem时...（Gem :: FilePermissionError）

Question

我收到了这个错误，我已经在互联网上寻找解决方案，但似乎没有工作。我正在使用rbenv。

Lachlans-MBP:~ MrTeale$ gem install rails -v 4.2.1
    Fetching: activesupport-4.2.1.gem (100%)
    ERROR:  While executing gem ... (Gem::FilePermissionError)
    You don't have write permissions for the /Library/Ruby/Gems/2.0.0 directory.