This is a gem housekeeping release. No user-facing changes.

🏠 Housekeeping

• Add functional test with vagrant on CI (#509) @colorbox
• Migrate CI build from Travis to GitHub Actions (#507) @colorbox
• Fix test by require 'set' (#508) @colorbox
• Bump toolmantim/release-drafter from 5.20.1 to 5.22.0 (#501, #502, #506) @dependabot

Full Changelog: https://github.com/capistrano/sshkit/compare/v1.21.3...v1.21.4