gnu-maintenance: Really find the latest GNOME releases.

gnu-maintenance: Really find the latest GNOME releases.

Reported by Efraim Flashner <efraim@flashner.co.il>
at <https://lists.gnu.org/archive/html/guix-devel/2015-12/msg00202.html>.

* guix/gnu-maintenance.scm (latest-ftp-release): Call KEEP-FILE? on
directories too.
(latest-gnome-release)[even-minor-version?]: Protect again MINOR being
false; change pattern to match VERSION with two or more numbers.  Remove
use of 'version-major+minor'.
[even-numbered-tarball?]: Rename to...
[even-numbered?]: ... this.  Use 'package-name->name+version'.  When
VERSION is #f, check NAME.