Implement Zone ID presence verification (GTFS Rule)

[maximearmstrong]

Is your feature request related to a problem? Please describe.
Zone IDs should be present in stops.txt if fare rules are defined in fare_rules.txt.

Describe the solution you'd like
Verify if rules are defined in fare_rules.txt. If yes, Zone IDs are required.

Describe alternatives you've considered
A clear and concise description of any alternative solutions or features you've considered.

Additional context
GTFS Spec http://gtfs.org/reference/static/#stopstxt