replace travis badge with gha

Author: joshday

README.md

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 | Build | Test |
 |-------|------|
-| [![Build Status](https://travis-ci.org/joshday/SweepOperator.jl.svg?branch=master)](https://travis-ci.org/joshday/SweepOperator.jl)  | [![codecov.io](http://codecov.io/github/joshday/SweepOperator.jl/coverage.svg?branch=master)](http://codecov.io/github/joshday/SweepOperator.jl?branch=master) |
+| [![CI](https://github.com/joshday/SweepOperator.jl/actions/workflows/CI.yml/badge.svg)](https://github.com/joshday/SweepOperator.jl/actions/workflows/CI.yml) | [![codecov.io](http://codecov.io/github/joshday/SweepOperator.jl/coverage.svg?branch=master)](http://codecov.io/github/joshday/SweepOperator.jl?branch=master) |
 
 
 The symmetric sweep operator is a powerful tool in computational statistics with uses in stepwise regression, conditional multivariate normal distributions, MANOVA, and more.  This package exports a single function: