Update README to reflect GitHub branch rename master -> main

Author: xiehan

README.md

@@ -2,7 +2,7 @@
 
 A PHP-based server-side proxy for interacting with the [NPR One API](https://dev.npr.org/api/)'s authorization server. Use this proxy to secure your OAuth2 credentials.
 
-[![Packagist](https://img.shields.io/packagist/v/npr/npr-one-backend-proxy.svg?maxAge=2592000)](https://packagist.org/packages/npr/npr-one-backend-proxy) [![Packagist](https://img.shields.io/packagist/l/npr/npr-one-backend-proxy.svg?maxAge=2592000)](https://github.com/npr/npr-one-backend-proxy-php/blob/master/LICENSE.md) [![Packagist](https://img.shields.io/packagist/dt/npr/npr-one-backend-proxy.svg?maxAge=2592000)](https://packagist.org/packages/npr/npr-one-backend-proxy) [![Build Status](https://travis-ci.org/npr/npr-one-backend-proxy-php.svg?branch=master)](https://travis-ci.org/npr/npr-one-backend-proxy-php) [![Coverage Status](https://coveralls.io/repos/github/npr/npr-one-backend-proxy-php/badge.svg?branch=master)](https://coveralls.io/github/npr/npr-one-backend-proxy-php?branch=master)
+[![Packagist](https://img.shields.io/packagist/v/npr/npr-one-backend-proxy.svg?maxAge=2592000)](https://packagist.org/packages/npr/npr-one-backend-proxy) [![Packagist](https://img.shields.io/packagist/l/npr/npr-one-backend-proxy.svg?maxAge=2592000)](https://github.com/npr/npr-one-backend-proxy-php/blob/main/LICENSE.md) [![Packagist](https://img.shields.io/packagist/dt/npr/npr-one-backend-proxy.svg?maxAge=2592000)](https://packagist.org/packages/npr/npr-one-backend-proxy) [![Build Status](https://travis-ci.org/npr/npr-one-backend-proxy-php.svg?branch=main)](https://travis-ci.org/npr/npr-one-backend-proxy-php) [![Coverage Status](https://coveralls.io/repos/github/npr/npr-one-backend-proxy-php/badge.svg?branch=main)](https://coveralls.io/github/npr/npr-one-backend-proxy-php?branch=main)
 
 ##### Table of Contents