ND-255 - Rename old Fare model and introduce new version of the Fare model

[woshilapin]

The first commit is only a renaming of struct so to simplify the reviewing process, you might want to look at each commit individually.

[datanel]

tests fail because of transit_model=0.5.0 which is not released yet

[woshilapin]

I also added the feature fare-v2 in the build (change of .travis.yml).

[woshilapin]

I removed the serde feature from the chrono crate in 6d9bc74.

[woshilapin]

I change the currency_code type from CurrencyCode to a String to avoid having our public API depending on a crate we're not sure is gonna stay. However, I still use this crate iso4217 to do the validation during serialization and deserialization (see 341a1fe).