Move development dependencies from the gemspec to the Gemfile #164

Follow-up to #161 (dependency policy).

The gemspec describes what the gem needs to run. Guard, RSpec, RuboCop, byebug and rake only matter to people developing the gem. Moving them to the Gemfile follows the Bundler recommendation and clears Gemspec/DevelopmentDependencies (9 offenses) from the .rubocop_todo.yml baseline.

Scope

• Move every add_development_dependency from ofx.gemspec to the Gemfile, keeping the same version constraints.
• Remove the Gemspec/DevelopmentDependencies entry from .rubocop_todo.yml.
• Document in the README Development section that development dependencies live in the Gemfile.