Integrate direnv with flakes (#503)

* Integrate direnv with flakes

This change makes local development easier for NixOS-based systems by
introducing a flake which declares the Ruby dependencies of this
project. This is useful in cases in which we don't want (or need) to
declare Ruby and it's dependencies globally.

* Do not track `.envrc

Author: saulecabrera

.gitignore

@@ -15,3 +15,15 @@ target/
 
 # rspec failure tracking
 .rspec_status
+
+# Emacs
+*~
+\#*\#
+.\#*
+
+# Direnv
+.direnv/
+.envrc
+
+# JIT dump
+*.dump

flake.lock

@@ -0,0 +1,16 @@
+ {
+    inputs.nixpkgs.url = "github:NixOS/nixpkgs/nixos-unstable";
+    inputs.flake-utils.url = "github:numtide/flake-utils";
+
+    outputs = { nixpkgs, flake-utils, ... }:
+      flake-utils.lib.eachDefaultSystem (system: {
+        devShells.default = nixpkgs.legacyPackages.${system}.mkShell {
+          packages = with nixpkgs.legacyPackages.${system}; [
+            ruby # Stable
+            bundler
+            libffi
+            llvmPackages_latest.libcxx
+          ];
+        };
+      });
+  }