blender: unbreak cudaSupport

Currently it errors out at build time with:

/nix/store/HASH-cudatoolkit-6.5.19/usr_include/host_config.h:82:2:
error: #error -- unsupported GNU version! gcc 4.9 and up are not supported!

Instead of downgrading gcc to 4.8, this patch upgrades cuda to 7.0, which
I think is the better choice. (Cuda 7 dropped support for some older graphics
cards, but gained support for newer ones.)

pkgs/applications/misc/blender/default.nix

···
 , libtiff, mesa, openal, opencolorio, openexr, openimageio, openjpeg, python
 , zlib, fftw
 , jackaudioSupport ? false, libjack2
-, cudaSupport ? false, cudatoolkit65
+, cudaSupport ? false, cudatoolkit7
 , colladaSupport ? true, opencollada
 }:
 
···
       opencolorio openexr openimageio /* openjpeg */ python zlib fftw
     ]
     ++ optional jackaudioSupport libjack2
-    ++ optional cudaSupport cudatoolkit65
+    ++ optional cudaSupport cudatoolkit7
     ++ optional colladaSupport opencollada;
 
   postUnpack =