Fix NervesHubChannel compile warning

farmbot_ext/lib/farmbot_ext/amqp/connection_worker.ex

@@ -140,7 +140,10 @@ defmodule FarmbotExt.AMQP.ConnectionWorker do
     {:reply, conn, state}
   end
 
-  defp open_connection(token, email, bot, mqtt_server, vhost) do
+  # Public function because the NervesHub channel requires it.
+  # TODO(Connor) - Fix that
+  @doc false
+  def open_connection(token, email, bot, mqtt_server, vhost) do
     Logger.info("Opening new AMQP connection.")
 
     # Make sure the types of these fields are correct. If they are not