Fix #350, specify JSON schema draft version

[davidlatwe]

Changes

• Specifying our schema draft version to version 4, because the module jsonschema we vendorized was only supported up to version 4.

• Change keyword const to enum, because const was introduced in JSON schema Draft 6.

You may see #350 for detail :)

[mottosso]

Nice one! For completeness, our vendored version of jsonschema won't actually take this into account, but once/if we update then these constraints should kick in? Or do they work already?

[davidlatwe]

Yeah, the keyword const was not working, for example, you could change the container id from pyblish.avalon.container into arbitrary string and it still pass the validation.

For example:

from avalon.vendor import jsonschema

data = {"foo": "bar"}
schema = {"properties": {"foo": {"const": "not-bar"}}}
jsonschema.validate(data, schema)
# No error raised

[mottosso]

Aaah so the current schema was using a too new keyword, and this brings it back to the version we're actually using to validate. Gotcha, that makes sense. Happy with this!