Vertical scroll snapping (fixes #393)

After a limited poke around the source, line 51 of src/snap.js seems to be the culprit.
Both el[i].offsetLeft and el[i-1].offsetLeft were 0 (as they are vertically aligned). Changing the comparison to <= fixes the issue.

I haven't checked this with a horizontal, or horizontal and vertical scrolling list, so may introduce other issues.

(fixes #393)