Example code error on Heroku

[jiaqixu]

I use the latest version of alpaca-trade-api (I have seen issue #12 and #15), it runs fine in local machine while still get error on Heroku

2019-08-13T17:27:46.655181+00:00 app[worker.1]: File "main.py", line 5, in
2019-08-13T17:27:46.655297+00:00 app[worker.1]: algo.main()
2019-08-13T17:27:46.655301+00:00 app[worker.1]: File "/app/samplealgo/algo.py", line 209, in main
2019-08-13T17:27:46.655453+00:00 app[worker.1]: price_df = prices(Universe)
2019-08-13T17:27:46.655456+00:00 app[worker.1]: File "/app/samplealgo/algo.py", line 61, in prices
2019-08-13T17:27:46.655570+00:00 app[worker.1]: return _get_prices(symbols, end_dt)
2019-08-13T17:27:46.655573+00:00 app[worker.1]: File "/app/samplealgo/algo.py", line 51, in _get_prices
2019-08-13T17:27:46.655683+00:00 app[worker.1]: return barset.df
2019-08-13T17:27:46.655686+00:00 app[worker.1]: File "/app/.heroku/python/lib/python3.6/site-packages/alpaca_trade_api/entity.py", line 102, in df
2019-08-13T17:27:46.655808+00:00 app[worker.1]: df = bars.df.copy()
2019-08-13T17:27:46.655811+00:00 app[worker.1]: File "/app/.heroku/python/lib/python3.6/site-packages/alpaca_trade_api/entity.py", line 84, in df
2019-08-13T17:27:46.655926+00:00 app[worker.1]: df.index * 1e9, utc=True,
2019-08-13T17:27:46.655929+00:00 app[worker.1]: File "/app/.heroku/python/lib/python3.6/site-packages/pandas/util/_decorators.py", line 208, in wrapper
2019-08-13T17:27:46.656074+00:00 app[worker.1]: return func(*args, **kwargs)
2019-08-13T17:27:46.656077+00:00 app[worker.1]: File "/app/.heroku/python/lib/python3.6/site-packages/pandas/core/tools/datetimes.py", line 785, in to_datetime
2019-08-13T17:27:46.656397+00:00 app[worker.1]: result = convert_listlike(arg, box, format)
2019-08-13T17:27:46.656401+00:00 app[worker.1]: File "/app/.heroku/python/lib/python3.6/site-packages/pandas/core/tools/datetimes.py", line 374, in _convert_listlike_datetimes
2019-08-13T17:27:46.656634+00:00 app[worker.1]: arg, _ = maybe_convert_dtype(arg, copy=False)
2019-08-13T17:27:46.656639+00:00 app[worker.1]: File "/app/.heroku/python/lib/python3.6/site-packages/pandas/core/arrays/datetimes.py", line 2032, in maybe_convert_dtype
2019-08-13T17:27:46.657262+00:00 app[worker.1]: data = data.astype(_NS_DTYPE)
2019-08-13T17:27:46.657266+00:00 app[worker.1]: File "/app/.heroku/python/lib/python3.6/site-packages/pandas/core/indexes/numeric.py", line 369, in astype
2019-08-13T17:27:46.657472+00:00 app[worker.1]: raise TypeError(msg)
2019-08-13T17:27:46.657492+00:00 app[worker.1]: TypeError: Cannot convert Float64Index to dtype datetime64[ns]; integer values are required for conversion