Update tileset schema to reflect required properties

[e-andersson]

Cesium now throws an exception when "asset": { "version": 0.0} is missing from tileset.json, so if that's the desired behavior it should be reflected in the schema. Judging from the example tileset.json https://github.com/AnalyticalGraphicsInc/3d-tiles#tilesetjson there may be other updates needed.

[pjcozzi]

What exactly needs to be updated?

asset and version are marked as required in the schema:

• https://github.com/AnalyticalGraphicsInc/3d-tiles/blob/master/schema/tileset.schema.json
• https://github.com/AnalyticalGraphicsInc/3d-tiles/blob/master/schema/asset.schema.json

[e-andersson]

Sorry, my mistake -- didn't see the separate asset schema...

[pjcozzi]

All good. Thanks for keeping us honest!