修改travis-ci徽标分支

README.md

 # NutUI 2
-![npm version](https://img.shields.io/npm/v/@nutui/nutui.svg)  [![Build Status](https://api.travis-ci.org/jdf2e/nutui.svg?branch=v2)](https://github.com/jdf2e/nutui/) [![Coverage Status](https://coveralls.io/repos/github/jdf2e/nutui/badge.svg?branch=master)](https://coveralls.io/github/jdf2e/nutui?branch=master) ![license](https://img.shields.io/npm/l/@nutui/nutui.svg)
+![npm version](https://img.shields.io/npm/v/@nutui/nutui.svg)  [![Build Status](https://api.travis-ci.org/jdf2e/nutui.svg?branch=master)](https://github.com/jdf2e/nutui/) [![Coverage Status](https://coveralls.io/repos/github/jdf2e/nutui/badge.svg?branch=master)](https://coveralls.io/github/jdf2e/nutui?branch=master) ![license](https://img.shields.io/npm/l/@nutui/nutui.svg)
  
 一套移动端轻量级Vue组件库