fix compile warning

mix.exs

@@ -14,7 +14,7 @@ defmodule TowerBugsnag.MixProject do
   # Run "mix help compile.app" to learn about applications.
   def application do
     [
-      extra_applications: [:logger],
+      extra_applications: [:logger, :public_key],
       env: [base_url: "https://notify.bugsnag.com", api_key: nil, environment: nil]
     ]
   end