optional agency_id

As @ragarwal6397 said:

>Per the specification document, the agency_id field is optional: https://developers.google.com/transit/gtfs/reference?csw=1#agency_txt___Field_Definitions

>However, this condition doesn't seem to be handled properly in the python scripts when trying to generate timetable.dat using one such dataset for the Chicago CTA - http://www.transitchicago.com/downloads/sch_data/google_transit.zip.

More info: https://github.com/bliksemlabs/rrrr/issues/122