Trigger the Unplugged notification on app start

... rather than waiting for a device to be added or removed.

This should have been implemented as part of the Celluloid conversion, but I forgot.
I removed the relevant comment that was intended to remind me.

Fixes #57.

app/models/sensor_watch.rb

@@ -34,11 +34,7 @@ def record_race_results(_, results)
  end

  def handle_device_change(_)
    if @sensor.plugged_in?
      initialize_state
    else
      self.state = :unplugged
    end
    initialize_state
  end

  def start_race
@@ -58,6 +54,7 @@ def quit
private

  def initialize_state
    return self.state = :unplugged unless @sensor.plugged_in?
    if @heat.current.any?
      start_race
    else

lib/track_sensor/base.rb

@@ -83,7 +83,6 @@ def initialize_device(device_path)

  def scan_for_device_changes
    initialize_new_devices
    # raise IOError.new('The sensor is not plugged in') unless plugged_in?
  end

  def initialize_new_devices