Crash if no columns are specified

If I have just 'csv_importable' alone, I get:

```
active_admin_csv_import/lib/active_admin_csv_import/dsl.rb:9:in `csv_importable': undefined local variable or method `active_admin_config' for #<ActiveAdmin::ResourceDSL:0x007fae20966a38> (NameError)
```