Update build widget

Using sheilds.io format to display the build status of the repo.

Author: namanbansal

README.md

@@ -1,6 +1,6 @@
 # Authorize.Net Python SDK 
 
-[![Build Status](https://travis-ci.org/AuthorizeNet/sdk-python.png?branch=master)]
+[![Travis](https://img.shields.io/travis/AuthorizeNet/sdk-python/master.svg)]
 (https://travis-ci.org/AuthorizeNet/sdk-python)
 [![Coverage Status](https://coveralls.io/repos/github/AuthorizeNet/sdk-python/badge.svg?branch=master)]
 (https://coveralls.io/github/AuthorizeNet/sdk-python?branch=master)