`xpp3` no longer optional in `dstu2016may`

[jamesnetherton]

Since the xpp3 groupId switch from xpp3 to org.ogce, <optional>true</optional> was removed from the dependency declaration in ca.uhn.hapi.fhir:org.hl7.fhir.dstu2016may:

https://github.com/hapifhir/org.hl7.fhir.core/commit/490e57fa7cabd114c8b3401c388f3e8c957322dc#diff-01401616945b253c0ca2df7d195b2a75f6f65d5ace5f6f7412bed091eb88f9a4

It causes some classloading issues for me due to xpp3 bundling its own QName class. It's simple enough to resolve by excluding xpp3. I just wondered whether it should be declared as an optional dependency like it was in the past.

[grahamegrieve]

@dotasek ?

[dotasek]

@jamesnetherton it looks to me like the omission of optional was made in error. It's not consistent with the other modules. I will test to see if adding it back in impacts anything, and likely make it optional again.