Fixed issue with duplicate thumbnails on ProductController#show

Spree 1.1.4 changes the way thumbnails were displayed. I didn't look into what exactly changed, but I think `Product#images` and `Product#variant_images` both returned the master variant image. This makes sense, but the thumbnail's partial was not updated to reflect this – in my case I was getting duplicate thumbnail display. This PR fixed the duplicate thumbnail display issue and cleans up the thumbnails partial.

Also, there were references to both `@product` and `product`, I switched to using the instance variable for consistency's sake.

This applies to master, 1-2-stable, and 1-1-stable.