iOS：解决pod的Insecure world writabl

当我们运行pod setup的命令的时候，有时候会碰到这个警告：

/Library/Ruby/Gems/2.0.0/gems/cocoapods-0.33.1/lib/cocoapods/executable.rb:55: warning: Insecure world writable dir /usr/local in PATH, mode 040777

要去除这个警告，我们需要更改/usr/local的权限，一句话：

sudo chmod go-w /usr/local