Only displaying distinct orders on admin/orders

On ``admin/orders`` we've added some more fields to filter results on billing country / countries (``bill_address_country_id_in``) and ordered variant(s) (``line_items_variant_id_in``). Thanks to the help of Ransack this is working out of the box, however, multiple results for a single order show up. This is due to the way SQL-joins are made.

Searching in the Ransack docs, I found out that adding a simple ``distinct: true`` to the ``results`` call in ``Spree::Admin::OrdersController`` ``index`` method would be sufficient to fix this issue.

*Source*: https://github.com/ernie/ransack#simple-mode (item no. 3)

Though we could easily fix this for ourselves by overriding the ``index`` method, I can imagine more developers could encounter this issue and fixing it by overriding a whole method isn't a great way to go.

*The code* (https://github.com/spree/spree/blob/master/backend/app/controllers/spree/admin/orders_controller.rb#L38):

*Before*:
```ruby
@orders = @search.result.includes([:user, :shipments, :payments]).page(params[:page]).per(params[:per_page] || Spree::Config[:orders_per_page])
```

*After*:
```ruby
@orders = @search.result(distinct: true).includes([:user, :shipments, :payments]).page(params[:page]).per(params[:per_page] || Spree::Config[:orders_per_page])
```

*Env*:
- Rails 3.2.14
- Currently running 2-0 stable, but the code in master is unchanged