Refactor to remove Rails dependency

[nimmolo]

Addresses issue #156

solid_queue does not appear to depend on the whole Rails stack.

# bin/rails

require "rails"
# Pick the frameworks you want:
require "active_model/railtie"
require "active_job/railtie"
require "active_record/railtie"
# require "active_storage/engine"
require "action_controller/railtie"
# require "action_mailer/railtie"
require "action_view/railtie"
# require "action_cable/engine"
require "rails/test_unit/railtie"
require "rails/engine/commands"

This PR reduces dependencies to the following, but maybe it only needs railties?

# solid_queue.gemspec
  rails_version = ">= 7.1"
  spec.add_dependency "activerecord", rails_version
  spec.add_dependency "activejob", rails_version
  spec.add_dependency "railties", rails_version

[rosa]

Hey @nimmolo, thanks for this and so sorry for the delay! I've been working on unrelated stuff the last couple of weeks 🙏