Data dumps are being tagged with the previous month's date

[vitorbaptista]

For example, a data dump that happened on 01/May/2017 would be tagged as 01/Apr/2017 (see https://airflow.opentrials.net/admin/airflow/log?execution_date=2017-04-01T00%3A00%3A00&task_id=dump_api_database&dag_id=data_dumps). The fix involves changing this to the correct value: https://github.com/opentrials/opentrials-airflow/blob/ec44992c82d25eb865f25c76adc1cace8bd8815a/dags/data_dumps.py#L60