Remove Appveyor (replace with github actions?)

[geneorama]

I think we need to remove Appveyor. The documentation (which is 4 years old) says to use usethis to initialize, but when I tested it in a new package I received the message that it is now out of date / deprecated.

If true, this will close #160

I don't think this affects the current submission #220 and #215

> usethis::use_appveyor()
✔ Setting active project to '/home/rstudio/testingappveyor'
Error: Cannot detect that project is already a Git repository.
Do you need to run `use_git()`?
In addition: Warning message:
`use_appveyor()` was deprecated in usethis 2.0.0.
ℹ Please use `use_github_actions()` instead.
This warning is displayed once every 8 hours.
Call `lifecycle::last_lifecycle_warnings()` to see where this warning was generated. 

[tomschenkjr]

That makes sense. Prior to GitHub Actions, CI/CD work was always handled by third parties like Appveyor (Windows) and Coveralls.io (Mac + Linux). GitHub Actions is well-supported and widely adopted so plenty of resources available to understand it.

[geneorama]

I thought Travis CI is for testing the Linux build and Coveralls.io is for measuring code coverage?

Travis CI was complaining about being over on credits, although support just said they unfreeze the account. I thought it would be worthwhile to at least get that working again.

Also, I think it would be sufficient to test in Ubuntu for our purposes, as I'm confident that if our simple R package works in Ubuntu it will most likely work on Windows or a Mac. Limiting the tests would be easier to manage and more reliable, while also staying with DevOps principles.

Do you think the additional tests are critical (@tomschenkjr or @nicklucius?)

[tomschenkjr]

You're right, I misremembered which products were doing which.

I would be cautious on removing OSes from the testing matrix. At times, certain dependencies act differently between the OS. CRAN tests on different OSes so it's a way to test ahead of time to avoid being scolded by CRAN moderators.