Fix file size comparator return value

With OC 7.0.2, sorting files and directories by size gives unpredictable results, as you can see [here](https://cloud.githubusercontent.com/assets/264472/4551756/53dccc9e-4e76-11e4-8d09-fc46fda697ac.png) and [here](https://cloud.githubusercontent.com/assets/264472/4551757/53dcfaca-4e76-11e4-958f-a8c321c89197.png).
This is due to the file size comparator function not doing what its documentation says ("return int -1 if $a must come before $b, 1 otherwise") - instead, it subtracts the two file sizes and returns the result.

After applying this simple, one-line change, sorting by file size works as expected for me.