Fix error on product cart form

When chosing a variant from option types and values, the input that was being
populated have a name of "product[#{id}]" when it should be "variant_id", at
least for version 2.3.
Merge pull request #3 from rwillrich/fix-error-on-product-cart-form

Fix error on product cart form
Adding ruby 2.1.2 and 2.1.3 to travis