Add minimal mix.exs

This should please mix(1) which should be able to build recon without
fetching rebar3 (even though it is already available system-wide).

Here is the failure reported by mix(1) otherwise in a unattended build:

    Shall I install rebar3? (if running non-interactively, use "mix local.rebar --force") [Yn]
    ** (Mix) Could not find "rebar3" to compile dependency :recon, please ensure "rebar3" is available

In `src/recon.app.src`, we also record `mix` as an acceptable build
tool. We also ensure `mix.exs` is uploaded to Hex.pm.

mix.exs

@@ -0,0 +1,13 @@
+defmodule Recon.MixProject do
+  use Mix.Project
+
+  def project do
+    [
+      app: :recon,
+      description: "Diagnostic tools for production use",
+      version: "2.4.0",
+      language: :erlang,
+      deps: []
+    ]
+  end
+end

src/recon.app.src

@@ -9,5 +9,6 @@
   {licenses, ["BSD"]},
   {links, [{"Github", "https://github.com/ferd/recon/"},
            {"Documentation", "http://ferd.github.io/recon/"}]},
-  {files, ["src/", "script/", "rebar.lock", "README.md", "LICENSE"]}
+  {build_tools, ["mix", "rebar3"]},
+  {files, ["src/", "script/", "rebar.lock", "mix.exs", "README.md", "LICENSE"]}
 ]}.