ci: test on Linux and Windows

[DanielRuf]

PR Checklist:

• I have run npm test locally and all tests are passing.
• I have added/updated tests for any new behavior.
  
• If this is a significant change, an issue has already been created where the problem / solution was discussed: [N/A, or add link to issue here]
  

PR Description

We can also test on Windows using Travis CI now and can remove AppVeyor then.

[reconbot]

Request is now in maintenance mode and wont be merging any new features. Please see #3142 for more information.