Deprecate numeric since ago

The user is expected to explicitly convert the value into an
AS::Duration, i.e. `5.ago` => `5.seconds.ago`

This will help to catch subtle bugs like:

```ruby
  def recent?(days = 3)
    self.created_at >= days.ago
  end
```

The above code would check if the model is created within the last 3
*seconds*.

In the future, Numeric#{ago,until,since,from_now} should be removed
completely, or throw some sort of errors to indicate there are no
implicit conversion from Numeric to AS::Duration.