cudatoolkit: switch the default from version 5 to 7

Version 5 doesn't build because it depends on broken/EOL'd python 2.6. Better
switch to something that works. Most users of unversioned cudatoolkit attr are
r-packages, but they are already marked as broken.

Another user is haskellPackages.cuda, it builds fine with cuda 7.

pkgs/top-level/all-packages.nix

@@ -1214,7 +1214,7 @@ let
     inherit callPackage;
   };
 
-  cudatoolkit = cudatoolkit5;
+  cudatoolkit = cudatoolkit7;
 
   curlFull = curl.override {
     idnSupport = true;