[Generators] ActiveJob

WIP !
I know I said that in general I prefer not to have version check flags. Though sometimes I feel that is an easier to understand and more general solution. I do think this way is a good idea. Though I'm wondering if a more generic type of metadata would be more beneficial.

Maybe something like;

```ruby
:min_rails_version => 4.2
```

Then we simple have a filter which excludes such specs if the running doesn't fit. Thoughts?