fixing exception in setting floating on undefined

One need use `.data("uiSortable")` instead of `.data("sortable")` in the current version of jQuery UI. The current code produces exception during the attempt to assign `floating` property on `undefined` object.

The exception will be catches for example in [the line](https://github.com/tonytomov/jqGrid/blob/v4.6.0/js/grid.base.js#L2661-L2663) of jqGrid code

    try {
        $(ts).jqGrid("sortableColumns", thr);
    } catch (e){}

but not in case of direct usage of `sortableColumns`. The method end always with exception and so the usage of `try {...} } catch (e){}` is really required in all usages of the current version of `sortableColumns` together with the current versions of jQuery UI.

The suggested fix should work on old and new versions of jQuery UI.
Merge pull request #618 from OlegKi/master

fixing exception at the end of sortableColumns