使用brew，gem安装工具的时候，总是容易报错，如下：
Fetching: aws-sdk-v1-1.67.0.gem (100%)
ERROR: While executing gem ... (Gem::FilePermissionError)
You don't have write permissions for the /Library/Ruby/Gems/2.3.0 directory.
解决：在命令前面加上sudo。
然后就会出现第二个错误：
Fetching: aws-sdk-v1-1.67.0.gem (100%)
ERROR: While executing gem ... (Gem::FilePermissionError)
You don't have write permissions for the /usr/bin directory.
解决：在命令的install 后面加上-n /usr/local/bin 即可成功。
