Installation request for topthink/think-captcha ^3.0 -> satisfiable by topthink/think-captcha[v3.0.0].

ThinkPHP5.1安装图形验证码的时候报错:

Problem 1
- Installation request for topthink/think-captcha ^3.0 -> satisfiable by topthink/think-captcha[v3.0.0].
- topthink/think-captcha v3.0.0 requires topthink/framework ^6.0.0 -> satisfiable by topthink/framework[6.0.x-dev, v6.0.0-rc2] but these conflict with your requirements or minimum-stability.

出错原因:
当我们使用命令 composer require topthink/think-captcha下载验证码库时，默认下载最新版本，当前最新版本是3.0，3.0版本是为ThinkPHP6准备的，所以我们需要下载2.0版本验证码库，2.0版本的才是为ThinkPHP5.1准备的，所以当我们使用composer命令下载验证码库的时候指定版本号即可

composer require topthink/think-captcha=2.*