fix(sortable): fix connected lists with helper for last spot drops

When using sortable between two lists and a helper function (not 'clone') there is a problem if you drop an item at the last spot.
The first time it works, the second time it will add it as the first element of the list.
It continues this way for every even drop as the last item.
You can check it here:
http://codepen.io/anon/pen/FyaJs

Closes #186