Switch tarball compression format to tar.xz only.
Switch tarball compression format to tar.xz only.

See http://mail.gnome.org/archives/gnome-announce-list/2011-September/msg00031.html
for more information.

https://bugzilla.gnome.org/show_bug.cgi?id=659140

choeger/pygobject-cmake,GNOME/pygobject,pexip/pygobject,davibe/pygobject,jdahlin/pygobject,jdahlin/pygobject,sfeltman/pygobject,GNOME/pygobject,choeger/pygobject-cmake,choeger/pygobject-cmake,davidmalcolm/pygobject,sfeltman/pygobject,davidmalcolm/pygobject,sfeltman/pygobject,thiblahute/pygobject,MathieuDuponchelle/pygobject,davibe/pygobject,davidmalcolm/pygobject,thiblahute/pygobject,MathieuDuponchelle/pygobject,GNOME/pygobject,thiblahute/pygobject,pexip/pygobject,pexip/pygobject,davibe/pygobject,jdahlin/pygobject,davibe/pygobject,MathieuDuponchelle/pygobject