all: add build badge

Author: changkun

README-zh-cn.md

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 # 现代 C++ 教程：高速上手 C++11/14/17/20
 
-[![](https://img.shields.io/badge/language-English-blue.svg?style=flat-square)](./README.md) [![](https://img.shields.io/badge/language-简体中文-red.svg?style=flat-square)](./README-zh-cn.md) [![](https://img.shields.io/badge/€-donate-ff69b4.svg?style=flat-square)](./assets/donate.md) [![](https://img.shields.io/badge/chat-community-667ed5.svg?style=flat-square)](./assets/community.md)
+![](https://img.shields.io/travis/changkun/modern-cpp-tutorial/master?style=flat-square) [![](https://img.shields.io/badge/language-English-blue.svg?style=flat-square)](./README.md) [![](https://img.shields.io/badge/language-简体中文-red.svg?style=flat-square)](./README-zh-cn.md) [![](https://img.shields.io/badge/€-donate-ff69b4.svg?style=flat-square)](./assets/donate.md) [![](https://img.shields.io/badge/chat-community-667ed5.svg?style=flat-square)](./assets/community.md)
 
 ## 本书目的
 

README.md

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 # Modern C++ Tutorial: C++11/14/17/20 On the Fly
 
-[![](https://img.shields.io/badge/language-English-blue.svg?style=flat-square)](./README.md) [![](https://img.shields.io/badge/language-简体中文-red.svg?style=flat-square)](./README-zh-cn.md) [![](https://img.shields.io/badge/€-donate-ff69b4.svg?style=flat-square)](./assets/donate.md) [![](https://img.shields.io/badge/chat-community-667ed5.svg?style=flat-square)](./assets/community.md)
+![](https://img.shields.io/travis/changkun/modern-cpp-tutorial/master?style=flat-square) [![](https://img.shields.io/badge/language-English-blue.svg?style=flat-square)](./README.md) [![](https://img.shields.io/badge/language-简体中文-red.svg?style=flat-square)](./README-zh-cn.md) [![](https://img.shields.io/badge/€-donate-ff69b4.svg?style=flat-square)](./assets/donate.md) [![](https://img.shields.io/badge/chat-community-667ed5.svg?style=flat-square)](./assets/community.md) 
 
 ## Purpose