[Generators] ActiveJob

WIP !
It's not just one spec. [We](https://github.com/rspec/rspec-rails/blob/209d27471f15310d4f7bc169988527760040df48/spec/generators/rspec/install/install_generator_spec.rb#L26-L42) [have](https://github.com/rspec/rspec-rails/blob/209d27471f15310d4f7bc169988527760040df48/spec/rspec/rails/matchers/relation_match_array_spec.rb#L8) [several](https://github.com/rspec/rspec-rails/blob/209d27471f15310d4f7bc169988527760040df48/spec/rspec/rails/example/view_example_group_spec.rb#L114-L118) [places](https://github.com/rspec/rspec-rails/blob/209d27471f15310d4f7bc169988527760040df48/spec/generators/rspec/scaffold/scaffold_generator_spec.rb#L68-L83) where we already do a version check. I'd prefer to think of a generic way, if possible to do cleanly, to generalize this. Also, we know other changes are coming done the line. It's just a question of when.