chore: switch main to master (#383)

Author: bshaffer

.github/workflows/tests.yml

@@ -2,7 +2,7 @@ name: Test Suite
 on:
   push:
     branches:
-      - master
+      - main
   pull_request:
 
 jobs:

README.md

@@ -1,4 +1,4 @@
-[![Build Status](https://travis-ci.org/firebase/php-jwt.png?branch=master)](https://travis-ci.org/firebase/php-jwt)
+[![Build Status](https://github.com/firebase/php-jwt/actions/workflows/tests.yml/badge.svg)
 [![Latest Stable Version](https://poser.pugx.org/firebase/php-jwt/v/stable)](https://packagist.org/packages/firebase/php-jwt)
 [![Total Downloads](https://poser.pugx.org/firebase/php-jwt/downloads)](https://packagist.org/packages/firebase/php-jwt)
 [![License](https://poser.pugx.org/firebase/php-jwt/license)](https://packagist.org/packages/firebase/php-jwt)