Run CI workflow in GitHub Actions

[chrisdambrosio]

A GitHub Actions workflow that replaces the tests that run on Travis CI.

GitHub Actions will enable contributors to run the tests on their own forked project without any additional configuration.

Runs test workers for the following configurations:

• Linux | OTP 24 | Elixir 1.12
• Linux | OTP 24 | Elixir 1.11
• Linux | OTP 23 | Elixir 1.11
• Linux | OTP 23 | Elixir 1.10
• Linux | OTP 22 | Elixir 1.9
• Windows | OTP 24 | Elixir 1.12
• Extra checks (docs, hex, mix format)

Additional notes:

• Two tests related to switching to passive mode are skipped while an issue with Windows is being investigated.
• Remove Appveyor & TravisCI configurations
• Update docs

[fhunleth]

This is awesome and solves a huge pain point for me when working on this project. Thank you!!