Fix badge, use master branch (#2538)

README.md

@@ -1,7 +1,7 @@
 # axios
 
 [![npm version](https://img.shields.io/npm/v/axios.svg?style=flat-square)](https://www.npmjs.org/package/axios)
-[![build status](https://img.shields.io/travis/axios/axios.svg?style=flat-square)](https://travis-ci.org/axios/axios)
+[![build status](https://img.shields.io/travis/axios/axios/master.svg?style=flat-square)](https://travis-ci.org/axios/axios)
 [![code coverage](https://img.shields.io/coveralls/mzabriskie/axios.svg?style=flat-square)](https://coveralls.io/r/mzabriskie/axios)
 [![install size](https://packagephobia.now.sh/badge?p=axios)](https://packagephobia.now.sh/result?p=axios)
 [![npm downloads](https://img.shields.io/npm/dm/axios.svg?style=flat-square)](http://npm-stat.com/charts.html?package=axios)