Return newest entry by date, not created_at

We only use this method in tests right now, but it's nice to be consistent with what we mean by "newest".
Does Demeter really apply for scopes? We're getting an object back with the same interface. :tongue: 

Like... if we had a coupon:

```ruby
class Coupon
  scope :unused,  -> { ... }
  scope :expired, -> { ... }
end
```

And I wanted to get all unused, expired coupons, isn't it alright to do:

```ruby
coupons.unused.expired
```

Or is there a better strategy for this kind of thing?