sort versions numerically

[laubster]

Addresses #96

[syohex]

Please don't use -V(--version-sort) option.-V(--version-sort) option is provided only by GNU coreutils sort. BSD sort which is installed in MacOSX, *BSD distributions does not provide such option. There is following error on MacOSX.

% plenv versions
  system
sort: invalid option -- V
Try `sort --help' for more information.

[ravenhall]

This would be a nice thing to have in plenv. +1