guile-2-test: Update to 2.0.

svn path=/nixpkgs/trunk/; revision=26059

pkgs/top-level/guile-2-test.nix

···
     allPackages {
       inherit system;
       config.packageOverrides = pkgs: {
-        guile = pkgs.guile_1_9;
+        guile = pkgs.guile_2_0;
       };
     };