CI Improvements

[tisba]

Inspired by: #276

• build and then install the gem and do a smoke test to see if it is in fact working.

  I removed *.c from the resulting gem bundle (🤦) in gemspec: Exclude *.bundle (follow-up #281) #285 and builds where still passing. IMO this should be easy to catch.

• install and smoke test with various ruby versions.

  In a relatively simple sanity check, we could test (e.g. using Docker) that mini_racer is installable and works with the "standard" Ruby docker images. These are pretty heavily used in the community and IMO it would be helpful to have some sort of confirmation that these work in general. We could start with ruby:3.0, ruby:3.1 and ruby:3.2 (see https://hub.docker.com/_/ruby).