improvement: Add :crypto to extra_applications

In order to fix a warning from recent elixir versions, list the erlang
:crypto module in mix.exs under `extra_applications`.

mix.exs

@@ -31,7 +31,7 @@ defmodule IP.Mixfile do
   # Run "mix help compile.app" to learn about applications.
   def application do
     [
-      extra_applications: [:logger]
+      extra_applications: [:crypto, :logger]
     ]
   end