remove appveyor, update CI

.github/workflows/CI.yml

@@ -12,7 +12,6 @@ jobs:
       fail-fast: false
       matrix:
         version:
-          - '1.6'
           - '1' # automatically expands to the latest stable 1.x release of Julia
           - 'nightly'
         os:

README.md

@@ -2,7 +2,7 @@
 
 | Build | Test |
 |-------|------|
-| [![Build Status](https://travis-ci.org/joshday/SweepOperator.jl.svg?branch=master)](https://travis-ci.org/joshday/SweepOperator.jl) [![Build status](https://ci.appveyor.com/api/projects/status/at5bcso64joc6wbj/branch/master?svg=true)](https://ci.appveyor.com/project/joshday/sweepoperator-jl/branch/master) | [![codecov.io](http://codecov.io/github/joshday/SweepOperator.jl/coverage.svg?branch=master)](http://codecov.io/github/joshday/SweepOperator.jl?branch=master) |
+| [![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) |
 
 
 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: