Add travis build status badge to readme

Summary:
The 7.8 build keeps breaking, but no one notices until someone opens an issue.
With the travis badge, breakage will at least be visible from the github
landing page for Haxl repo.

Reviewed By: zilberstein

Differential Revision: D3656468

fbshipit-source-id: 5537ab78b9dd44a5a3f07e7e3db7f73ec959f096

readme.md

@@ -45,3 +45,5 @@ Graph API.
    Hackage.
 
  * [There is no Fork: An Abstraction for Efficient, Concurrent, and Concise Data Access](http://community.haskell.org/~simonmar/papers/haxl-icfp14.pdf), our paper on Haxl, accepted for publication at ICFP'14.
+
+[![Build Status](https://travis-ci.org/facebook/Haxl.svg?branch=master)](https://travis-ci.org/facebook/Haxl)