fhir.resources has changed

[jonahbenton]

Looks like fhir.resources has changed with version 6:

$ python DSTU2.py smart_it_sandbox.json
Traceback (most recent call last):
  File "/home/tcp/src/github.com/the-commons-project/commonpass_sample_resources/DSTU2.py", line 16, in <module>
    from fhir.resources.DSTU2.fhirreference import FHIRReference
ModuleNotFoundError: No module named 'fhir.resources.DSTU2.fhirreference'

Fixed with requirements.txt:
fhir.resources==5.1.1