Decommission Travis build & set up AppVeyor Linux build

[craigfowler]

Builds in the Travis CI environment no longer function since they ended easy support for Open Source projects. They still support Open Source but it is complex to configure. AppVeyor supports Linux now so I'd like to switch to AppVeyor for both Windows & Linux build & test.

To Reproduce

Steps to reproduce the behavior:

1. Push new code changes to master or a branch
2. Visit the Travis & AppVeyor build systems

Expected behavior

I'd like to see new builds for both Windows & Linux in AppVeyor. I don't want to see any builds in Travis anymore.

Actual behavior

I only see Windows builds in AppVeyor, I don't see any builds in Travis (since they stopped building some years ago).