README: Add badge for Travis CI.

README.md

 # docker-stacks
 
+[![Build Status](https://travis-ci.org/jupyter/docker-stacks.svg?branch=master)](https://travis-ci.org/jupyter/docker-stacks)
 [![Join the chat at https://gitter.im/jupyter/jupyter](https://badges.gitter.im/Join%20Chat.svg)](https://gitter.im/jupyter/jupyter?utm_source=badge&utm_medium=badge&utm_campaign=pr-badge&utm_content=badge)
 
 Opinionated stacks of ready-to-run Jupyter applications in Docker.