Readme: add Travis-CI badge so people can easily see there is tests (#646)

* Readme: add Travis-CI badge so people can easily see there is tests

Following https://twitter.com/MoOx/status/776008513252392960

* Readme: add Travis-CI badge so people can easily see there is tests, round 2

README.md

-# Create React App
+# Create React App [![Build Status](https://travis-ci.org/facebookincubator/create-react-app.svg?branch=master)](https://travis-ci.org/facebookincubator/create-react-app)
 
 Create React apps with no build configuration.