* I upgraded nixpkgs to the latest master which now contains
libtensorflow. This replaces the custom tensorflow-c. Note that
libtensorflow also builds on OS X and has optional CUDA support.
* nixpkgs is now pinned using builtins.fetchTarball which means we
can build with an empty NIX_PATH thus making the build pure.
This does require Nix-2.0 but this has been released for a while
with NixOS-18.03.
* The stack override has been replaced by passing
`extraArgs = "--system-ghc";` to buildStackProject.
