Deprecated Numeric#{ago,until,since,from_now}

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`.

/cc @jeremy @sikachu