Separately update zbus to 1.9.2 to fix cargo audit issue (#59)

This was done using `cargo update zbus --precise=1.9.2`, since `cargo
update` did only update to 1.9.1.

Also note that now runing `cargo update` will *downgrade* zbus again to 1.9.1:

    % cargo update
	Updating crates.io index
	Updating bitflags v1.2.1 -> v1.3.2
	  Adding cfg-if v0.1.10
	Removing memoffset v0.6.4
	Updating nix v0.20.2 -> v0.17.0
	  Adding void v1.0.2
	Updating zbus v1.9.2 -> v1.9.1
	Updating zbus_macros v1.9.2 -> v1.9.1

See also the following cargo issues:
 * rust-lang/cargo#7671
 * rust-lang/cargo#5702