use getAttribute('href') on img elements instead of img.href

In the rel-group preloading function, the "href" attribute of a Colorbox image is referenced via `i.href` (where `i` is the image element object). However, at least in Firefox 18 and Chromium 23, the `href` property doesn't appear to be defined for image element objects. So use `getAttribute('href')` instead to get that property.