fluxus: switch to racket_7_9

pkgs/applications/graphics/fluxus/default.nix

···
 , ode
 , openal
 , openssl
-, racket
+, racket_7_9
 , sconsPackages
 , zlib
 }:
···
     openssl
     zlib
   ];
+  racket = racket_7_9;
 in
 stdenv.mkDerivation rec {
   pname = "fluxus";
···
     ode
     openal
     openssl.dev
-    racket
+    racket_7_9
   ];
   nativeBuildInputs = [ sconsPackages.scons_3_1_2 ];