Use the Travis badge from the main branch

The `main` branch is the new default.

README.md

@@ -2,7 +2,7 @@
   <a href="https://github.com/gsamokovarov/jump/releases">[releases]</a>
 </p>
 
-[![Build Status](https://travis-ci.org/gsamokovarov/jump.svg?branch=master)](https://travis-ci.org/gsamokovarov/jump) [![Go Report Card](https://goreportcard.com/badge/github.com/gsamokovarov/jump)](https://goreportcard.com/report/github.com/gsamokovarov/jump)
+[![Build Status](https://travis-ci.org/gsamokovarov/jump.svg?branch=main)](https://travis-ci.org/gsamokovarov/jump) [![Go Report Card](https://goreportcard.com/badge/github.com/gsamokovarov/jump)](https://goreportcard.com/report/github.com/gsamokovarov/jump)
 
 **Jump** integrates with your shell and learns about your navigational habits by
 keeping track of the directories you visit. It gives you the most visited