fix build badge on readme

Author: ablaom

README.md

@@ -4,6 +4,6 @@ Repository implementing MLJ interface for
 [DecisionTree](https://github.com/bensadeghi/DecisionTree.jl) models.
 
 
-[![Build Status](https://travis-ci.com/alan-turing-institute/MLJDecisionTreeInterface.jl.svg?branch=master)](https://travis-ci.com/github/alan-turing-institute/MLJDecisionTreeInterface.jl)
+[![Build Status](https://github.com/alan-turing-institute/MLJDecisionTreeInterface.jl/workflows/CI/badge.svg)](https://github.com/alan-turing-institute/MLJDecisionTreeInterface.jl/actions)
 [![Coverage](https://coveralls.io/repos/github/alan-turing-institute/MLJDecisionTreeInterface.jl/badge.svg?branch=master)](http://codecov.io/github/alan-turing-institute/MLJDecisionTreeInterface.jl?branch=master)
 [![Code Style: Blue](https://img.shields.io/badge/code%20style-blue-4495d1.svg)](https://github.com/invenia/BlueStyle)