text: Title: get_batch_stock_quotes stopped working
  Body: Hi!
  
  I was using get_batch_stock_quotes on regular basis and suddenly it stopped working (without updating av or any other python libraries). Calling the function for some list of stocks produces the error: 
  
    File ".../site-packages/alpha_vantage/alphavantage.py", line 292, in _handle_api_call
      raise ValueError(json_response["Error Message"])
  ValueError: This API function (BATCH_STOCK_QUOTES) does not exist.
  
  I used simple code:
  ts = TimeSeries(key='myPremiumKey',
                  output_format='pandas', 
                  indexing_type='date')
  ts.get_batch_stock_quotes(['AAPL','MSFT','AMZN','GOOG'])
  
  Do you have any idea what might be a problem?