我在 mac 下用终端执行 gem install appium_lib 命令,被告知

ERROR:  While executing gem ... (Gem::FilePermissionError)
    You don't have write permissions for the /Library/Ruby/Gems/2.0.0 directory.

然后我使用 sudo 命令,依旧不成功

Building native extensions.  This could take a while...
ERROR:  While executing gem ... (Errno::EPERM)
    Operation not permitted - /usr/bin/nokogiri

大神能帮忙看看到底是为什么吗?