Add build badge

Author: ValentinH

README.md

@@ -3,6 +3,7 @@
 Status:
 [![npm version](https://img.shields.io/npm/v/angularjs-slider.svg?style=flat-square)](https://www.npmjs.com/package/angularjs-slider)
 [![npm downloads](https://img.shields.io/npm/dm/angularjs-slider.svg?style=flat-square)](http://npm-stat.com/charts.html?package=angularjs-slider&from=2015-01-01)
+[![Build Status](https://travis-ci.org/angular-slider/angularjs-slider.svg?branch=master)](https://travis-ci.org/angular-slider/angularjs-slider)
 
 Links:
 [![Join the chat at https://gitter.im/rzajac/angularjs-slider](https://badges.gitter.im/Join%20Chat.svg)](https://gitter.im/rzajac/angularjs-slider?utm_source=badge&utm_medium=badge&utm_campaign=pr-badge&utm_content=badge)