kde4: use older ruby to fix qtruby build

/cc maintainer @ttuegel. Discussion:
https://github.com/NixOS/nixpkgs/pull/12610#issuecomment-188666473

I'm personally not certain that building bindings against a non-default
ruby version is a good thing, but it's likely better than a build failure.

pkgs/top-level/all-packages.nix

@@ -14845,6 +14845,7 @@ let
           perlBindings = true;
           pythonBindings = true;
         };
+        ruby = ruby_2_2; # see https://github.com/NixOS/nixpkgs/pull/12610#issuecomment-188666473
       }
       ../desktops/kde-4.14;