docs: Added build badge

Author: FezVrasta

README.md

@@ -1,5 +1,6 @@
 ## React Popper
 
+[![Build Status](https://travis-ci.org/FezVrasta/react-popper.svg?branch=master)](https://travis-ci.org/FezVrasta/react-popper)
 [![npm version](https://img.shields.io/npm/v/react-popper.svg)](https://www.npmjs.com/package/react-popper)
 [![npm downloads](https://img.shields.io/npm/dm/react-popper.svg)](https://www.npmjs.com/package/react-popper)
 [![Dependency Status](https://david-dm.org/souporserious/react-popper.svg)](https://david-dm.org/souporserious/react-popper)