Ajaxify image

*Issue*: There is an issue in the [ajaxify.js] (https://github.com/Shopify/Timber/blob/master/assets/ajaxify.js.liquid), when the default cart is used (useCartTemplate = false).
If the product that is added to the cart has no image, the ajaxify shopping cart doesn't open.

*Reason*: There is a [hack] (https://github.com/Shopify/Timber/blob/master/assets/ajaxify.js.liquid#L797-804) in this code that create the thumbnail of the image by adding "_small" at the end of the file name. However, /cart.js will return null for products with no image.

*Good Solution*: Change the Shopify API to return the "no-image" thumbnail instead of null when there is no image assigned to the product

*Hack Solution*: hard-coding the url of the default no-image thumbnail. I just used [the one] (http://cdn.shopify.com/s/assets/admin/no-image-medium-cc9732cb976dd349a0df1d39816fbcc7.gif) that I got from my shop. It doesn't look to be the generic one, and also it is medium according to the url. 

Edit: I added some minor syntax fixes in the last commit. All the rest was done by @afshing