Fix spec failure due to big directory size on travis
Fix spec failure due to big directory size on travis

Listing the fixtures folder sorting by size, expected the files at the beginning
followed by the `symlinks` folder. But on travis, the folder is 4KiB which is
larger than any one of the files.

Keep the folder first by adding the `--group-directories-first` flag.

athityakumar/colorls,athityakumar/colorls