cast DS to timestamp (#117)

https://github.com/mozilla/docker-etl/pull/116 was intended to fix a typing issue when uploading to BQ, but the fix did not work as expected. This fix _should_ work; I created a BQ table to reproduce the error and verify this fix.

jobs/kpi-forecasting/kpi-forecasting/Utils/DBWriter.py

@@ -23,7 +23,7 @@ def write_predictions_to_bigquery(
     predictions["target"] = config["target"]
     predictions["forecast_date"] = today
     predictions["forecast_parameters"] = str(forecast_parameters)
-    predictions["ds"] = pd.to_datetime(predictions["ds"])
+    predictions["ds"] = pd.to_datetime(predictions["ds"], utc=True)
 
     output_table = config["output_table"]