[chore] Make the build status badge point towards master (#497)

Author: darrachequesne

README.md

@@ -1,7 +1,7 @@
 
 # Engine.IO client
 
-[![Build Status](https://secure.travis-ci.org/socketio/engine.io-client.svg)](http://travis-ci.org/socketio/engine.io-client)
+[![Build Status](https://travis-ci.org/socketio/engine.io-client.svg?branch=master)](http://travis-ci.org/socketio/engine.io-client)
 [![NPM version](https://badge.fury.io/js/engine.io-client.svg)](http://badge.fury.io/js/engine.io-client)
 
 This is the client for [Engine.IO](http://github.com/socketio/engine.io),