Setup GitHub Actions and update dependencies

[okuramasafumi]

Using travis-ci.org as a CI service is not possible now. We can migrate to travis-ci.com or GitHub Actions, so it's an attempt to use GitHub Actions as a CI service. To run bundle exec rake some dependencies must be updated. After updating RSpec some modifications are required to suppress deprecation warnings.

[okuramasafumi]

@mitchellh Hi, this change is just for running CI and doesn't change its behavior. Merging this would help other developers to work on this gem.