gajim: Revert update to latest mercurial branch.

This reverts commit 83ba20511bf553d2dd395a6fe4af73fc9d86d57c.

We now have nbxmpp in upstream <nixpkgs>, so we no longer need it in our
own overrides and there was a new tarball release issued upstream just
one day after we did that override, so let's move over to using this one
instead.

Signed-off-by: aszlig <aszlig@redmoonstudios.org>

Conflicts:
    overrides/default.nix
    pkgs/gajim/0.16-pre.nix
    pkgs/nbxmpp/default.nix
i3: Allow to work only with a single head.

This is very ugly and just to be taken as a workaround because the i3
status line will be completely messed up. The main reason for doing this
is that the config will evaluate with a single head.

Signed-off-by: aszlig <aszlig@redmoonstudios.org>
machines: Add arilou, an USB pendrive machine.

I was keeping this one around for some time, but it's really time to
finally add it to my nixconf repository so Hydra builds the machine as
well.

Signed-off-by: aszlig <aszlig@redmoonstudios.org>