dropbox: 38.4.27 -> 40.4.46

pkgs/applications/networking/dropbox/default.nix

···
 # Dropbox client to bootstrap installation.
 # The client is self-updating, so the actual version may be newer.
 let
-  version = "38.4.27";
+  version = "40.4.46";
 
   arch = {
     "x86_64-linux" = "x86_64";