nix: Update nixpkgs and static-haskell-nix

nix/overlays/default.nix

@@ -5,4 +5,5 @@
   gitignore = import ./gitignore.nix;
   haskell-packages = import ./haskell-packages.nix;
   postgresql-default = import ./postgresql-default.nix;
+  postgresql-legacy = import ./postgresql-legacy.nix;
 }

nix/overlays/postgresql-legacy.nix

@@ -0,0 +1,20 @@
+self: super:
+# Overlay that adds legacy versions of PostgreSQL that are supported by
+# PostgREST.
+{
+  # PostgreSQL 9.5 was removed from Nixpkgs with
+  # https://github.com/NixOS/nixpkgs/commit/72ab382fb6b729b0d654f2c03f5eb25b39f11fbb
+  # We pin its parent commit to get the last version that was available.
+  postgresql_9_5 =
+    let
+      rev = "55ac7d4580c9ab67848c98cb9519317a1cc399c8";
+      tarballHash = "02ffj9f8s1hwhmxj85nx04sv64qb6jm7w0122a1dz9n32fymgklj";
+
+      pinnedPkgs =
+        builtins.fetchTarball {
+          url = "https://github.com/nixos/nixpkgs/archive/${rev}.tar.gz";
+          sha256 = tarballHash;
+        };
+    in
+    (import pinnedPkgs { }).pkgs.postgresql_9_5;
+}