text: Title: Handle dash.exceptions.PreventUpdate
  Body: As reported in #205, when a callback raises PreventUpdate the wrapper code does not handle it.
  
  A change in [views.py](https://github.com/GibbsConsulting/django-plotly-dash/blob/master/django_plotly_dash/views.py) should allow these exceptions to be handled. The `update` function needs to catch this exception and return an empty response with a status code of 204; this is how the underlying [dash](https://github.com/plotly/dash/blob/2d735aa250fc67b14dc8f6a337d15a16b7cbd6f8/dash/dash.py#L381) code handles this response.
  