undefined method `group' for nil:NilClass

Version 1.3.1 creates problems on rails 3.2.18.

``` ruby
@klass.where(conditions).includes(options[:include]).distinct(options[:distinct]) # => 
```

returns nil.
Because of the nil value I get "undefined method `group' for nil:NilClass"

on https://github.com/saulabs/reportable/blob/master/lib/saulabs/reportable/report.rb#L127
