Use bundle exec and add Github Actions cache

[ghost]

Bundler

Instead of rake use bundle exec rake and setup bundler to install gems in vendor/bundle which will be cached by Github Actions.

Changelog

• Fix toolchain:lint for vendor/bundle
• Update Gemfile
• Fix YAML
• Fix YAML
• Add github actions cache
• Add missing test-unit dependency
• Explicitly install bundler
• Use bundle instead of setup.sh and pure rake