Update submodules: plugins (new commits)

plugins:
    f6c68e2 Greg Holmes <greg@dmdirc.com>
            Remove relaybot plugin, can be moved externally if still wanted

            Gerrit: http://gerrit.dmdirc.com/3192

Change-Id: I707c192727cc8b6d7f5e215b65d4f75f6b1b356e