Update to JSON Schema Draft 07, new jsonschema2md version

[trieloff]

I've updated jsonschema2md to include support for draft 07 (it still has support for draft-06, but you need the -v 06 flag) and it might make sense to update XDM to the latest schema version (and update the tooling as well)

[kstreeter]

@trieloff is there anything in draft-7 that is necessary? This is actually a very fundamental change, and will have a lot of impact on our requirements and testing around XDM validation.

[trieloff]

According to the change logs linked in http://json-schema.org/specification-links.html#draft-7 there are no functional changes in draft 07, just a clarification of language. It might still make sense to upgrade, as most of the underlying tooling will upgrade sooner or later.