Update setup script using latest setup_script_generator
Update setup script using latest setup_script_generator

Particularly, the setup script now not only works with asdf but also
prefers it over rbenv for installing Ruby.

[ci skip]

thoughtbot/shoulda-matchers,thoughtbot/shoulda-matchers,thoughtbot/shoulda-matchers,biow0lf/shoulda-matchers,reacuna/shoulda-matchers,biow0lf/shoulda-matchers,reacuna/shoulda-matchers,biow0lf/shoulda-matchers,thoughtbot/shoulda-matchers,reacuna/shoulda-matchers,biow0lf/shoulda-matchers,reacuna/shoulda-matchers