Update README.md

Author: lazytiger

README.md

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 # Trojan-rs
-[![Build Status](https://img.shields.io/travis/lazytiger/trojan-rs)](https://travis-ci.com/lazytiger/trojan-rs)
+[![Build Status](https://travis-ci.com/lazytiger/trojan-rs.svg?branch=master)](https://travis-ci.com/lazytiger/trojan-rs)
 
 ***[Trojan](https://github.com/trojan-gfw/trojan) server and proxy programs written in Rust.***