Added the travis ci build status back into the README.

README.md

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 ![Alamofire: Elegant Networking in Swift](https://raw.githubusercontent.com/Alamofire/Alamofire/assets/alamofire.png)
 
+[![Build Status](https://travis-ci.org/Alamofire/Alamofire.svg)](https://travis-ci.org/Alamofire/Alamofire)
+
 Alamofire is an HTTP networking library written in Swift.
 
 ## Features