18.09: let's call it "Jackrabbit"

I was mainly considering Jellyfish and Jaguar (and Jackrabbit).
Originally I was inclined for Jellyfish, but then I thought of the
release T-shirts someone makes and it didn't seem suitable...
Jaguar would keep the name referring to a car as well, but as a
not-too-old (Mac) OS version is codenamed that way, I didn't go for it.

nixos/modules/misc/version.nix

@@ -86,7 +86,7 @@ in
       versionSuffix = mkIf (pathIsDirectory gitRepo) (mkDefault (".git." + gitCommitId));
 
       # Note: code names must only increase in alphabetical order.
-      codeName = "Impala";
+      codeName = "Jackrabbit";
     };
 
     # Generate /etc/os-release.  See