update readme with build status

Author: melode11

README.md

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 # Socket.IO C++ Client
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+[![Build Status](https://travis-ci.org/socketio/socket.io-client-cpp.svg)](https://travis-ci.org/socketio/socket.io-client-cpp)
 This repository contains the Socket.IO C++ client. It depends on [websocket++](https://github.com/zaphoyd/websocketpp) and is inspired by [socket.io-clientpp](https://github.com/ebshimizu/socket.io-clientpp).
 
 By virtue of being written in C++, this client works in several different platforms. The [examples](https://github.com/socketio/socket.io-client-cpp/tree/master/examples) folder contains an iPhone, QT and Console example chat client!