github-default-branch: Update README.md

Author: mheap

README.md

@@ -1,6 +1,6 @@
 # OpenTok Java SDK
 
-[![Build Status](https://travis-ci.org/opentok/Opentok-Java-SDK.svg?branch=master)](https://travis-ci.org/opentok/Opentok-Java-SDK)
+[![Build Status](https://travis-ci.org/opentok/Opentok-Java-SDK.svg?branch=main)](https://travis-ci.org/opentok/Opentok-Java-SDK)
  [![Contributor Covenant](https://img.shields.io/badge/Contributor%20Covenant-v2.0%20adopted-ff69b4.svg)](CODE_OF_CONDUCT.md)
 
 <img src="https://assets.tokbox.com/img/vonage/Vonage_VideoAPI_black.svg" height="48px" alt="Tokbox is now known as Vonage" />