feat(build): Add Nix build configuration

Adds a Nix build configuration baesd on the default rustPlatform
configuration.

The configuration pins a specific nixpkgs commit (which at the time
was a commit tracking nixpkgs-unstable).

.travis.yml

@@ -1 +1 @@
-language: rust
+language: nix

default.nix

@@ -0,0 +1,31 @@
+{ pkgs ? import <nixpkgs> {}
+, doCheck ? true }:
+
+# This pins the nixpkgs version to an - admittedly arbitrarily chosen
+# - commit with the correct dependencies.
+# This should be removed once the majority of relevant machines are
+# running NixOS 18.03.
+
+let unstable = import (pkgs.fetchFromGitHub {
+  owner  = "NixOS";
+  repo   = "nixpkgs-channels";
+  rev    = "ea145b68a019f6fff89e772e9a6c5f0584acc02c";
+  sha256 = "18jr124cbgc5zvawvqvvmrp8lq9jcscmn5sg8f5xap6qbg1dgf22";
+}) {};
+in with unstable; rustPlatform.buildRustPackage rec {
+  name        = "converse-${version}";
+  version     = "0.1.0";
+  src         = ./.;
+  cargoSha256 = "0a0634v22wazmyym05x4ricqkxwn4r3spv6s2f3ilma65cy6qyrf";
+
+  buildInputs = [ openssl pkgconfig ];
+
+  inherit doCheck;
+
+  meta = with stdenv.lib; {
+    description = "A simple forum software";
+    homepage    = "https://github.com/tazjin/converse";
+    license     = licenses.gpl3;
+    maintainers = [ maintainers.tazjin ];
+  };
+}