retina-image() mixin causes compile error in compass

The retina-image() mixin is currently commented out in order to get scss to compile.

Line 66 of sass/_mixins.scss: Invalid CSS after "  $at2x_path: ": expected expression (e.g. 1px, bold), was "~`${path}.repla...")

Using Compass version: 1.0.1

This can easily be fixed by replacing with a known working retina image mixin written in SASS/SCSS.
