rename module to avoid collision when subclasses inherits from AR::Base

I had:
```ruby
class User < ActiveRecord::Base
  acts_as_messageable
  
  class Profile < ActiveRecord::Base
  end
end
```
Resulting in `uninitialized constant Mailboxer::Models::Messageable::ActiveRecord::Base` because ActiveRecord exists in the first ancestor: `Mailboxer::Models::Messageable`

I could have done `class Profile < ::ActiveRecord::Base` but well...