Refresh bugfix

A small bugfix.

A bit of background. I was calling refresh (without any arguments) after editing the skrollr `data-` attributes on a bunch of elements. On a few elements, I'd removed the skrollr attributes all together, such that skrollr should no longer be managing those elements. However, their ``___skrollable_id`` properties were being left on them in the DOM. The result was that, when skrollr looped through the elements with ``___skrollable_id``s in order to build the internal object for each one, it found multiple DOM elements with the same ``___skrollable_id`` in some cases (as the id counter had been reset). This caused the keyframe objects to get messed up, with the empty keyframe properties from the now-unmanaged elements sometimes overwriting the legitimate keyframe properties other element with the same id.