text: Title: Allow pandas series as value to `info_box_content`
  Body: The following should work:
  
  ```py
  import gmaps
  import gmaps.datasets
  gmaps.configure(api_key="AI...")
  
  df = gmaps.datasets.load_dataset_as_df('starbucks_kfc_uk').sample(200)
  
  fig = gmaps.figure()
  marker_layer = gmaps.marker_layer(
      df[['latitude', 'longitude']],
      info_box_content=df['chain_name']
  )
  fig.add_layer(marker_layer)
  fig
  ```
  
  At the moment, we have to call `.values` on `df['chain_name']` for this example to work. See issue #200.