Remove NOT NULL constraint from trips.direction_id

This field is not required, and can have a null state:
https://developers.google.com/transit/gtfs/reference#trips_direction_id_field
Update gtfs-parser build dependencies

See comments in build.sbt for install instructions
Import calendars, agencies, trips, stops via gtfs-parser

TODO added for shapes once the SRID issue is resolved,
  commented out code provided
Merge pull request #19 from WorldBank-Transport/feature/load-gtfs-data

Load GTFS Data via gtfs-parser