Prefer direnv's use_nix to shell.nix

I find it bothersome to share the list of Haskell dependencies between my
default.nix and shell.nix files. A few days ago, I created a THIRD file,
shared.nix, that defined the shared code b/w default.nix and shell.nix. This
DRY'd things up, but it also added a new file, which I didn't like.

Today I learned that direnv integrates with Nix using a function called
`use_nix`. Voila! I typically already have .envrc files per-project, so this
doesn't add any unnecessary files, and it allows me to delete my shell.nix
files.

I would use `lorri`, except that I encountered issues using Lorri on my work
computer, which I'm not interested in attempting to resolve now.

zoo/.envrc

@@ -0,0 +1 @@
+use_nix

zoo/shell.nix

@@ -1,10 +0,0 @@
-let
-  briefcase = import /home/wpcarro/briefcase {};
-in briefcase.buildHaskell.shell {
-  deps = hpkgs: with hpkgs; [
-    hspec
-    rio
-    string-conversions
-    servant-server
-  ];
-}