Based on: https://github.com/ruby/setup-ruby
Re: #60
Some notes on the initial Github Actions config:
1. Set min ruby version to 2.5 in order to support JRuby,
which as of 9.2.17.0 has a RUBY_VERSION of 2.5.
2. For CodeCov exclude JRuby and TruffleRuby, to avoid
errors sending the results in Github Actions on those
jobs rather than debugging them further.
3. Plan is to next remove Travis CI, and then to convert
the CodeCov config to use the Github Action for CodeCov
(thanks @taichi-ishitani for this suggestion!)
Marc Siegel