gnu: emacs-which-key: update to 3.4.0

[[PGP Signed Part:No public key for 7C7AFFBEFEF2CB25 created at 2020-03-17T19:10:10+0100 using RSA]]
* gnu/packages/emacs-xyz.scm (emacs-which-key): Update to 3.4.0

From f36fee740bab3d74b526358be421a1527af9b69d Mon Sep 17 00:00:00 2001
From: Michael Rohleder <mike@rohleder.de>
Date: Tue, 17 Mar 2020 19:03:45 +0100
Subject: [PATCH] gnu: emacs-which-key: update to 3.4.0

* gnu/packages/emacs-xyz.scm (emacs-which-key): Update to 3.4.0