Update Travis CI badge to SVG

Author: Bryant Luk

README.md

@@ -2,7 +2,7 @@
 
 ![Home Image](https://raw.githubusercontent.com/wiki/paypal/PayPal-PHP-SDK/images/homepage.jpg)
 
-[![Build Status](https://travis-ci.org/paypal/PayPal-PHP-SDK.png?branch=master)](https://travis-ci.org/paypal/PayPal-PHP-SDK) 
+[![Build Status](https://travis-ci.org/paypal/PayPal-PHP-SDK.svg?branch=master)](https://travis-ci.org/paypal/PayPal-PHP-SDK)
 [![Coverage Status](https://coveralls.io/repos/paypal/PayPal-PHP-SDK/badge.svg?branch=master)](https://coveralls.io/r/paypal/PayPal-PHP-SDK?branch=master)
 
 __Welcome to PayPal PHP SDK__. This repository contains PayPal's PHP SDK and samples for REST API.