add a build status badge to the README

Summary:
Now that we have Travis CI builds enabled and passing, add a link to the build
status to the README.

Reviewed By: yfeldblum

Differential Revision: D6903206

fbshipit-source-id: 643b72a63a4113e00ff1602b1c3638384115b23d

README.md

 Folly: Facebook Open-source Library
 -----------------------------------
 
+[![Build Status](https://travis-ci.org/facebook/folly.svg?branch=master)](https://travis-ci.org/facebook/folly)
+
 ### What is `folly`?
 
 Folly (acronymed loosely after Facebook Open Source Library) is a