dev: Rails rubocop config, and drop support for ruby `< 2.5`

[flavorjones]

In this PR:

• import .rubocop.yml from rails/rails
• move development dependencies from gemspec to Gemfile
• set required_ruby_version to ">= 2.5.0"
• correct all rubocop warnings

Note that Ruby 2.5 matches the oldest supported version of Rails (6.0/6.1).