fix: fixes 'warning: :public_key.cacerts_get/0 is undefined'

mix.exs

@@ -14,7 +14,7 @@ defmodule TowerRollbar.MixProject do
   # Run "mix help compile.app" to learn about applications.
   def application do
     [
-      extra_applications: [:logger]
+      extra_applications: [:logger, :public_key]
     ]
   end