docs: changed broken link travis (#121)

* Update README.md

Author: Olexandr88

README.md

@@ -1,7 +1,7 @@
 Lambda-Term
 ===========
 
-[![Build Status](https://travis-ci.org/ocaml-community/lambda-term.svg?branch=master)](https://travis-ci.org/ocaml-community/lambda-term)
+[![CI](https://github.com/ocaml-community/lambda-term/actions/workflows/ci.yml/badge.svg)](https://github.com/ocaml-community/lambda-term/actions/workflows/ci.yml)
 
 Lambda-Term is a cross-platform library for manipulating the
 terminal. It provides an abstraction for keys, mouse events, colors,