undefined method `sub' for Sequel adapter with sqlite memory

I'm using a sqlite memory adapter in Sequel. When I try and run DB cleaner, it says:

```
gems/database_cleaner-0.8.0/lib/database_cleaner/sequel/truncation.rb:12:in `clean': undefined method `sub' for nil:NilClass (NoMethodError)
```

This is because there is no URI for the memory mode, I'd imagine.
