Update Gitter URL

Author: wzyboy

README.md

@@ -1,7 +1,7 @@
 # LAIN
 
 [![MIT license](https://img.shields.io/github/license/mashape/apistatus.svg)](https://opensource.org/licenses/MIT)
-[![Gitter](https://badges.gitter.im/gitterHQ/gitter.svg)](https://gitter.im/laincloud)
+[![Gitter](https://badges.gitter.im/gitterHQ/gitter.svg)](https://gitter.im/laincloud/opensource)
 
 Lain 是一个基于 docker 的 PaaS 系统。