Fix paper trail compatibility

Hello! 

This is a proposed solution to issue #206, for the case where a model has both `has_paper_trail` and `translates :versioning => true`

I've added the case to the set of tests. 

The code will only delegate #version and #versions to the translation class if the model doesn't already have paper_trail enabled. 

I believe there will be a problem if you define your model like this:

````
class Paper < ActiveRecord::Base
  translates :description, :versioning => true
  has_paper_trail :only => [:name]
````

The problem will be that `#version` and `#versions` will call the _translation_ methods. 

If you define it like this:
````
class Paper < ActiveRecord::Base
  has_paper_trail :only => [:name]
  translates :description, :versioning => true
````

It should work alright. 

I understand it's not ideal to have to order the statements, but I wanted to keep the contribution backwards compatible. 

With this contribution, if you want to have `has_paper_trail` and `translates :versioning => true` in the same model, you will have to access versions like this:
````
# for object's paper trail
obj.versions
# for object's translations paper trail
obj.translation.versions
````

Any comments will be appreciated. 

I hope this helps.

Thanks!