Update ajaxcart.js

Normally the Magento HTTPS frontends anchors links using HTTP. This behaviour crashes ajax requests. Browsers throws this message:

`The page at 'https://www.example.com/index.html' was loaded over HTTPS, but displayed insecure content from 'http://www.example.com/index.html?options=cart&ajax=true': this content should also be loaded over HTTPS.`

To fix you can replace location protocol before Ajax.Requests as proposed.