wrapAround and Parallax

Hi! There is an issue when activating both wrapAround and parallax. The overall project works, but parallax values are incorrect for the edges, making the first-1 and last+1 pictures not to show while scrolling (they will appear once the scroll is over).

**Test case:** http://codepen.io/iambz/pen/XjZVRX

Start by reproducing the behavior in the linked CodePen with both wrapAround and parallax enabled, then trace the Flickity code handling scrolling and edge cells. Done means the first-1 and last+1 pictures appear in the correct positions throughout scrolling, not only after scrolling stops.

Written by the indexing model from the issue text.