add license and travis shields to readme

README.md

+[![Build Status](https://travis-ci.org/amueller/word_cloud.png)](https://travis-ci.org/amueller/word_cloud)
+[![licence](http://img.shields.io/badge/licence-MIT-blue.svg?style=flat)](https://github.com/amueller/word_cloud/blob/master/LICENSE)
+
 word_cloud
 ==========