3.0.0.rc1

We've made a lot of changes recently, mostly around fixing newer bundler functionality at the expense of older ones. We'll try out this rc1 release for a while so that we can happy it's all working as expected.

• Fix error when using Appraisal and install_if (#226)
• Setup GitHub Actions for CI
• Fix: make .strip_heredoc private
• Update the code style across the project (#242)
• Cleanup gemspec (#241)
• Move development gems in Gemfile (#240)
• Ignore MacOS files (#239)
• Remove useless file (#238)
• Silence Bundler warnings (#237)
• Improve debug output (#236)
• Skip broken tests (#235)
• Split bundler dir from gems build dir (#234)
• Disable RSpec monkey patching (#233)
• Add rspec and bundler binstub (#232)
• Move appraisal script to 'exe' directory (#231)
• Cleanup Bundler ENV variable introduced in Bundler 2.4 (#229)
• Add dynamic-security.yml and SECURITY.md (#225)
• Add the ability to use variables in Gemfiles (#223)
• Fix extra new line when customising gemfiles (#222)
• Avoid using the --without parameter to Bundler (#221)
• Avoid using the --path parameter to Bundler (#220)
• Add support for ruby file: Gemfile syntax (#219)
• Make tests more resilient in different git environments (#215)
• Fix remove_gem example (#217)