Jigoshop update hid some "Add to cart" buttons

Basically the $button_type was set to 'cart' for some reason, possibly an older version of Jigoshop set this value.

A $button_type of 'cart' will ouput NO button. This caused a lot of angry users of my theme. Here is a fix.

More details in this forum thread:

http://forum.jigoshop.com/discussions/problems/9870-add-to-cart-buttons-have-disappeared