github-default-branch: Update README.md

Author: mheap

README.md

@@ -1,7 +1,7 @@
 # Nexmo Spring Boot Starter
 
 [![Maven Central](https://img.shields.io/maven-central/v/com.nexmo/nexmo-spring-boot-starter.svg?label=Maven%20Central)](https://search.maven.org/search?q=g:%22com.nexmo%22%20AND%20a:%22nexmo-spring-boot-starter%22)
-[![Build Status](https://travis-ci.org/Nexmo/nexmo-spring-boot-starter.svg?branch=master)](https://travis-ci.org/Nexmo/nexmo-spring-boot-starter)
+[![Build Status](https://travis-ci.org/Nexmo/nexmo-spring-boot-starter.svg?branch=main)](https://travis-ci.org/Nexmo/nexmo-spring-boot-starter)
 [![codecov](https://codecov.io/gh/Nexmo/nexmo-spring-boot-starter/branch/master/graph/badge.svg)](https://codecov.io/gh/Nexmo/nexmo-spring-boot-starter)
 
 <img src="https://developer.nexmo.com/assets/images/Vonage_Nexmo.svg" height="48px" alt="Nexmo is now known as Vonage" />