fixed undefined method version_key for ActiveRecord::Associations::JoinD...

The missing version_key method error can be simulated by running a simple join on versions

Ex.: `Address.joins(:versions).where("unit_number = ':n'", :n => 1000).to_a`

Where the address and the custom version classes are defined as follows

```ruby
class Address < ActiveRecord::Base
  has_paper_trail :class_name => 'AddressVersion', :on => [:update], :meta => {:unit_number => :unit_number, :house_number => :house_number, :house_suffix => :house_suffix}
end

class AddressVersion < Version
  self.table_name = :address_versions
  attr_accessible :unit_number, :house_number, :house_suffix, :event, :object, :whodunnit
end
```