assimp: Broken on Darwin

http://hydra.nixos.org/build/38315116

pkgs/development/libraries/assimp/default.nix

···
     homepage = http://assimp.sourceforge.net/;
     license = licenses.bsd3;
     maintainers = with maintainers; [ ehmry ];
-    platforms = platforms.unix;
+    platforms = platforms.linux;
   };
 }