Natural sort RUBIES by version.

- Uses the `find` command for building the `RUBIES` array.
  - Uses `-mindepth` and `-maxdepth` for better portability than `-d`.
- Uses a variation of mpapis's sed/sort code from #277 for natural version sorting (closes #277).
  - Dictionary sorts by Ruby name then numeric sorts by MAJOR, MINOR, and then TINY versions.
  - Patch versions are incidentally correctly ordered by the find command. (No additional sort collumn because existing patch versions sort correctly, there aren't new patch versions coming because of semantic versioning, and a simple numeric sort is ineffective due to the "p" prefix.)