Add fallback for Utils.compile_env/3

To allow for setups running a newer version of appsignal-elixir with
older versions of appsignal-phoenix or appsignal-plug, add a
deprecated fallback in order to not break any apps.

[changeset skip]

lib/appsignal/utils.ex

@@ -30,4 +30,9 @@ defmodule Appsignal.Utils do
     require Logger
     Logger.warning(message)
   end
+
+  @deprecated "Use Application.compile_env/3 instead."
+  def compile_env(app, key, default) do
+    Application.compile_env(app, key, default)
+  end
 end