Fix update of the thumbnails when selecting a variant

Hi,

in the rails4 branch I've found a problem where the thumbnails/product image was not updated when clicking on the variant.

I found it was due to two problems:
- the variantId received by the showVariantImages didn't match the attributes of the thumbnail.
- in the javascript, the thumbnail to display was selected using its class attribute, but in the markup the id attribute is the one to include the ID of the variant.