README: fix CI badge (#563)

Previous badge pointed to Travis, which is long gone.

Author: woodruffw

README.md

@@ -2,7 +2,7 @@ ruby-macho
 ================
 
 [![Gem Version](https://badge.fury.io/rb/ruby-macho.svg)](http://badge.fury.io/rb/ruby-macho)
-[![Build Status](https://travis-ci.org/Homebrew/ruby-macho.svg?branch=master)](https://travis-ci.org/Homebrew/ruby-macho)
+[![CI](https://github.com/Homebrew/ruby-macho/actions/workflows/tests.yml/badge.svg)](https://github.com/Homebrew/ruby-macho/actions/workflows/tests.yml)
 [![Coverage Status](https://codecov.io/gh/Homebrew/ruby-macho/branch/master/graph/badge.svg)](https://codecov.io/gh/Homebrew/ruby-macho)
 
 A Ruby library for examining and modifying Mach-O files.