Fixed sorting of osm changesets

The sorting function for the osm stream data is erroneous:

    function (a, b) { return (+new Date(a.meta.tilestamp)) - (+new Date(a.meta.tilestamp)); }

`b` is never used, instead `a` is used twice, resulting in a sort value which is always 0. The correct function is probably (note the `b` in the subtrahend):

    function (a, b) { return (+new Date(a.meta.tilestamp)) - (+new Date(b.meta.tilestamp)); }