Fixed missing import of time

[mahadi]

When removing a sensor from the bus, reading it would result in an import error in _update_sensor_data:

Traceback (most recent call last):
  File "ow_monitor.py", line 18, in <module>
    main()
  File "ow_monitor.py", line 13, in main
    t = ds.temperature()
  File "/usr/local/lib/python3.5/dist-packages/sensor/DS18B20.py", line 40, in temperature
    self._update()
  File "/usr/local/lib/python3.5/dist-packages/sensor/__init__.py", line 103, in _update
    self._update_callback(**kwargs)
  File "/usr/local/lib/python3.5/dist-packages/sensor/__init__.py", line 63, in locked
    func(*args, **kwargs)
  File "/usr/local/lib/python3.5/dist-packages/sensor/DS18B20.py", line 52, in _update_sensor_data
    time.sleep(0.2)
NameError: name 'time' is not defined