Add Travis build status badge

Author: tomdale

README.md

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 # FastBoot Express Middleware
 
+[![Build Status](https://travis-ci.org/ember-fastboot/fastboot-express-middleware.svg?branch=master)](https://travis-ci.org/ember-fastboot/fastboot-express-middleware)
+
 This middleware is a small wrapper around the
 [fastboot](https://github.com/ember-fastboot/fastboot) package, which
 renders Ember.js apps in Node.js.