[README] Use master branch for the travis build status

README.md

@@ -1,4 +1,7 @@
-#[Material-UI](http://callemall.github.io/material-ui/) [![npm package](https://img.shields.io/npm/v/material-ui.svg?style=flat-square)](https://www.npmjs.org/package/material-ui) [![Build Status](https://img.shields.io/travis/callemall/material-ui.svg?style=flat-square)](https://travis-ci.org/callemall/material-ui) [![Gitter](https://img.shields.io/badge/gitter-join%20chat-f81a65.svg?style=flat-square)](https://gitter.im/callemall/material-ui?utm_source=badge&utm_medium=badge&utm_campaign=pr-badge&utm_content=badge)
+#[Material-UI](http://callemall.github.io/material-ui/)
+[![npm package](https://img.shields.io/npm/v/material-ui.svg?style=flat-square)](https://www.npmjs.org/package/material-ui)
+[![Build Status](https://travis-ci.org/callemall/material-ui.svg?branch=master)](https://travis-ci.org/callemall/material-ui)
+[![Gitter](https://img.shields.io/badge/gitter-join%20chat-f81a65.svg?style=flat-square)](https://gitter.im/callemall/material-ui?utm_source=badge&utm_medium=badge&utm_campaign=pr-badge&utm_content=badge)
 
 [![PeerDependencies](https://img.shields.io/david/peer/callemall/material-ui.svg?style=flat-square)](https://david-dm.org/callemall/material-ui#info=peerDependencies&view=list)
 [![Dependencies](https://img.shields.io/david/callemall/material-ui.svg?style=flat-square)](https://david-dm.org/callemall/material-ui)