Added uniq to prevent duplications of records

I am glad this part got some attention before I was patching globalize like this:

```ruby
module Globalize::ActiveRecord::ClassMethods
  def with_translations(*locales)
    if locales.empty?
      includes(:translations).references(:translations) # my code
    else
      preload(:translations) # original code
        .joins(:translations)
        .readonly(false)
        .with_locales(locales)
        .with_required_attributes
    end
  end
end
```

However the new way causes duplicated records to appear when using `model.with_translations`, I propose to add `uniq` at least. In fact it was perfect like I had it with `includes(:translations).references(:translations)` but I see new version adds a bit more to the preload.