installed_perls sort

Previously commands like `list` `exec` iterated over perls in ASCII order (or maybe filesystem order?). i.e. perl-5.8.0 goes after perl-5.19.1

this fix make it sorted in ascending order by version.

if it's OK, I can continue and add some tests + fix `available_perls` similar way.
(note that `exec` with `--with` argument can be executed in order defined by `--with`)