add Travis CI build badge

Author: mohsinulhaq

README.md

@@ -1,6 +1,7 @@
 # React Tooltip
 [![npm version](https://img.shields.io/npm/v/react-popper-tooltip.svg?style=flat-square)](https://www.npmjs.com/package/react-popper-tooltip)
 [![npm downloads](https://img.shields.io/npm/dm/react-popper-tooltip.svg?style=flat-square)](https://www.npmjs.com/package/react-popper-tooltip)
+[![Build Status](https://img.shields.io/travis/com/mohsinulhaq/react-popper-tooltip.svg?colorB=purple&style=flat-square&logo=travis)](https://travis-ci.com/mohsinulhaq/react-popper-tooltip)
 [![Dependency Status](https://img.shields.io/david/mohsinulhaq/react-popper-tooltip.svg?style=flat-square)](https://david-dm.org/mohsinulhaq/react-popper-tooltip)
 [![code style: prettier](https://img.shields.io/badge/code_style-prettier-ff69b4.svg?style=flat-square)](https://github.com/prettier/prettier)
 ![PRs Welcome](https://img.shields.io/badge/PRs-welcome-brightgreen.svg?style=flat-square)