SDK-874: Add build button to README

Author: davidgrayston

README.md

@@ -1,5 +1,7 @@
 # Yoti PHP SDK
 
+[![Build Status](https://travis-ci.org/getyoti/yoti-php-sdk.svg?branch=master)](https://travis-ci.org/getyoti/yoti-php-sdk)
+
 Welcome to the Yoti PHP SDK. This repo contains the tools you need to quickly integrate your PHP back-end with Yoti, so that your users can share their identity details with your application in a secure and trusted way.
 
 ## Table of Contents