guix package: '--upgrade' no longer reverses package order.

Reported by Oleg Pykhalov <go.wigust@gmail.com>
at <https://lists.gnu.org/archive/html/help-guix/2017-10/msg00000.html>.

* guix/scripts/package.scm (options->installable)[upgraded]: Use
'fold-right' instead of 'fold'.