Compatibility with problem-1.0.1.yaml

[chicobento]

Detailed Description

org.zalando.problem.Problem javadocs are documented according with old 1.0.0 specification, specially the absolute requirement on URI fields.
It should be updated according with the new 1.0.1 specification.
Im not an expert, but I guess is that it could also impact other parts of the library, such as the ConstraintViolationProblem class since it points to an absolute URL hosted at zalando.github.io. Not sure how would be the best approach here.

Context

The "absolute" mentions on the URI fields goes against the 176 guideline:
Note: The use of absolute URIs is not forbidden but strongly discouraged. If you use absolute URIs, please reference problem-1.0.0.yaml#/Problem instead.

[whiskeysierra]

@tkrop 🏓