在安装fir-cli时出现了如下错误：
$ gem install fir-cli
Fetching: thor-0.20.0.gem (100%)
ERROR: While executing gem ... (Gem::FilePermissionError)
You don't have write permissions for the /Library/Ruby/Gems/2.3.0 directory.
解决办法：
sudo gem install -n /usr/local/bin fir-cli