remove Travis CI (since we have GitHub actions)

Author: ranocha

.travis.yml

@@ -2,7 +2,6 @@
 
 [![license](https://img.shields.io/github/license/mashape/apistatus.svg?maxAge=2592000)](https://github.com/JuliaMath/IterativeSolvers.jl/blob/master/LICENSE)
 [![Build Status](https://github.com/JuliaMath/IterativeSolvers.jl/workflows/CI/badge.svg)](https://github.com/JuliaMath/IterativeSolvers.jl/actions)
-[![Build Status on Linux](https://travis-ci.org/JuliaMath/IterativeSolvers.jl.svg?branch=master)](https://travis-ci.org/JuliaMath/IterativeSolvers.jl)
 [![Codecov](http://codecov.io/github/JuliaMath/IterativeSolvers.jl/coverage.svg?branch=master)](http://codecov.io/github/JuliaMath/IterativeSolvers.jl?branch=master)
 [![Coveralls](https://coveralls.io/repos/JuliaMath/IterativeSolvers.jl/badge.svg?branch=master&service=github)](https://coveralls.io/github/JuliaMath/IterativeSolvers.jl?branch=master)