Add GitHub Actions workflow

[mishina2228]

Since June 15th, 2021, the building on travis-ci.org is ceased.
So this PR adds GitHub Actions workflow.
Here are the details:

• Add Ruby 3.0 and 3.1 to the CI matrix
  • CI with Ruby 3.1 fails due to ruby-growl.
    As mentioned in Drop the use of deprecated ruby-growl gem #58, we should drop the use of it.
• Add rexml to Gemfile for Ruby 3.0+
  • rexml is a bundled gem from Ruby 3.0.
    Please see https://www.ruby-lang.org/en/news/2020/12/25/ruby-3-0-0-released/ for more info.
• Address CI failure in Ruby 3.0+
  • Since rspec-mocks v3.10.3, #with now distinguishes between keyword args and hash in Ruby 3.
    See Make with support Ruby 3 keywords rspec/rspec-mocks#1394 for details.