Fixed incorrect treatment of 'last' in float-span

I was trying to improve Scott's StackOverflow [answer](http://stackoverflow.com/a/15864326/901944) with this code:

    .column {
      @include float-span(1);
    
      &:last-child {
        @include float-span(1, 'last'); }}

And i found it not to be working! The last item is treated erroneously:

    .column:last-child {
      width: 6.38298%;
      clear: right;
      float: left;
      margin-left: 0%;
      margin-right: 2.12766%;
    }

Should be:

    .column:last-child {
      width: 6.38298%;
      clear: right;
      float: right;
      margin-right: 0%;
    }

I investigated the issue and found this code:

    @if type-of($grid) == 'number' and length($grid) == 1 {
      @if $location == 'last' or $location == 'omega' {
        $location: length($grid) - $span + 1;

The error is in the last line. The $grid is a number, and it's list length is always 1. We want the number itself, not the 1.

So the correct line would be:

        $location: $grid - $span + 1;

Pull baby, pull! :D