Fix variant thumbnails not showing on frontend

The thumbnails for variants would not show up on a different variant click, and the
product page opened without any visible thumbnails. They all stayed hidden.

This is on Spree 2.0 branch. 
It occurred when a product had 2 variants and added an image to each variant.

I changed the thumbnails ID to use the variant ID since is the one used on the variant checkboxs.

Hope the PR is ok. 
Regards!