Bypass is_external check when data-at2x is provided

The whole reason we bail on `this.is_external()` in `check_2x_variant()` is because checking for an image cross-domain via AJAX throws an error (see issue #3).

Recently, @ronny added the `data-at2x` attribute (see #66). This is great because it allows us to specify 2x paths, and the current implementation doesn't check whether these images exist or not. Therefore, there's no need to bail on external image paths when the `at_2x_path` is provided.

This change will provide a working solution for images hosted on CDNs, issue #40.