Added Travis CI Badge

Author: Jos Hickson

README.md

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 # Winton.Extensions.Threading.Actor
 
+[![Build Status](https://travis-ci.org/wintoncode/Winton.Extensions.Threading.Actor.svg?branch=master)](https://travis-ci.org/wintoncode/Winton.Extensions.Threading.Actor)
+
 A lightweight implementation of the actor pattern designed to integrate with C#'s `async`/`await` keywords.
 It is a richer version of the implementation outlined on [Winton's Tech Blog](https://tech.winton.com/blog/2017/03/a-tpl-actor-pattern).