Consistency: schemaurl uses URI-reference, protobuf uses URI-reference

[cneijenhuis]

Closes #384

In #169 it was decided that inside the specification, we only use URI-reference. (From reading the PR, it is not clear to me whether it was intended to change both schemaurl and source, or only source, but the PR effectively changed both).

In #338 I renamed the shorthand URI to URI-reference, because it was easy to wrongly assume only URIs are supported, but not URI references.
I missed changing the schemaurl type.
When schemaurl was introduced to spec.json, that was mixed up as well.

Related, the protobuf-format.md type mapping wasn't consistent with the type naming in spec.md (I think both PRs ran concurrently), which I'm fixing as well.

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There is probably a discussion to be had whether #169 actually intended to change schemaurl - the constraint suggests otherwise - and if we want to have both URI and URI-reference in the type system.

Signed-off-by: Christoph Neijenhuis christoph.neijenhuis@commercetools.de

[duglin]

Approved on the 2/7 call.