README: Update Travis badge URL to use .com

Author: olleolleolle

README.md

@@ -4,7 +4,7 @@ Provides useful `RSpec.shared_context`s for testing code that builds on top of [
 
 [async]: https://github.com/socketry/async
 
-[![Build Status](https://secure.travis-ci.org/socketry/async-rspec.svg)](http://travis-ci.org/socketry/async-rspec)
+[![Build Status](https://travis-ci.com/socketry/async-rspec.svg?branch=master)](https://travis-ci.com/socketry/async-rspec)
 [![Code Climate](https://codeclimate.com/github/socketry/async-rspec.svg)](https://codeclimate.com/github/socketry/async-rspec)
 [![Coverage Status](https://coveralls.io/repos/socketry/async-rspec/badge.svg)](https://coveralls.io/r/socketry/async-rspec)