在用composer安装验证码模块时，提示下面这些英文，不知道什么意思，哪位大神帮忙看下，为什么装不上。

Content-Length mismatch, received 21457 bytes out of the expected 853521
http://packagist.org could not be fully loaded, package information was loaded from the local cache and may be out of date
Using version ^2.0 for topthink/think-captcha
./composer.json has been updated
Loading composer repositories with package information
Updating dependencies (including require-dev)
Your requirements could not be resolved to an installable set of packages.

Problem 1
- topthink/think-captcha v2.0.2 requires topthink/framework 5.1.* -> satisfiable by topthink/framework[5.1.x-dev, v5.1-beta.1, v5.1-rc.1, v5.1-rc.2, v5.1-rc.3, v5.1.0, v5.1.1, v5.1.10, v5.1.11, v5.1.12, v5.1.13, v5.1.2, v5.1.3, v5.1.4, v5.1.5, v5.1.6, v5.1.7, v5.1.8, v5.1.9] but these conflict with your requirements or minimum-stability.
- topthink/think-captcha v2.0.1 requires topthink/framework 5.1.x-dev -> satisfiable by topthink/framework[5.1.x-dev] but these conflict with your requirements or minimum-stability.
- topthink/think-captcha v2.0 requires topthink/framework 5.1.x-dev -> satisfiable by topthink/framework[5.1.x-dev] but these conflict with your requirements or minimum-stability.
- Installation request for topthink/think-captcha ^2.0 -> satisfiable by topthink/think-captcha[v2.0, v2.0.1, v2.0.2].


Installation failed, reverting ./composer.json to its original content.


图片上上个红框里是安装图像处理模块时的返回结果，下一个红框是安装验证码模块时的提示。

到底该怎么操作才能安装成功啊？？要不网站一直提示

求助！求助！求助！！