Modernization updates/maintenance

[mjankowski]

Misc changes:

• Add bin/setup script to get running
• Remove travis, add GH actions
• Update min ruby version in gemspec, and min rails version in dev deps
• Remove cucumber/aruba spec approach, replace with (faster) combustion internal app approach (coverage preserved)
• Once over on documentation
• Lint with standard
• Minor naming/method refactors, prior behaviour preserved

[mjankowski]

I see there have been previous PRs/issues around lack of maintainer here.

I'd love to get this or something like it merged, and maybe a version bump (at minimum, clear up the ruby version dependency). I'll leave this open for a bit and if still no maintainer reply, considering publishing gem (with new name) from my fork.