Migrate CI from Travis to GitHub Actions

[mishina2228]

Since June 15th, 2021, the building on travis-ci.org is ceased. This PR migrates CI from Travis CI to GitHub Actions. Here are the details:

• Changed SCP from module to class in lib/net/scp/version.rb
  • bundle exec rake test raises the following error.

    /.../net-scp/lib/net/scp/upload.rb:3:in `<module:Net>': SCP is not a class (TypeError)
    /.../net-scp/lib/net/scp/version.rb:2: previous definition of SCP was here

• Update gem dependencies (bundler and rake)
• Fix deprecation warnings

refs: #59 (It can be closed after merging this PR)

[mishina2228]

I've added Ruby 3.2 to the CI matrix and suppressed Mocha deprecation warnings.