update coverage badge

README.md

@@ -2,7 +2,7 @@
 
 [![Join the chat at https://gitter.im/v2ray/v2ray-core](https://badges.gitter.im/Join%20Chat.svg)](https://gitter.im/v2ray/v2ray-core?utm_source=badge&utm_medium=badge&utm_campaign=pr-badge&utm_content=badge)
 [![Build Status](https://travis-ci.org/v2ray/v2ray-core.svg?branch=master)](https://travis-ci.org/v2ray/v2ray-core)
-[![Coverage Status](https://coveralls.io/repos/v2ray/v2ray-core/badge.svg?branch=master&service=github)](https://coveralls.io/github/v2ray/v2ray-core?branch=master)
+[![codecov.io](https://codecov.io/github/v2ray/v2ray-core/coverage.svg?branch=master)](https://codecov.io/github/v2ray/v2ray-core?branch=master)
 [![GoDoc](https://godoc.org/github.com/v2ray/v2ray-core?status.svg)](https://godoc.org/github.com/v2ray/v2ray-core)
 
 V2Ray 是一个模块化的代理软件包，它的目标是提供常用的代理软件模块，简化网络代理软件的开发。