Provide validation support for 1.0.0-beta.1

Presently 1.0.0-beta.1 stac objects try to validate against schemas.stacspec.org, for example: 

https://schemas.stacspec.org/v1.0.0-beta.1/item-spec/json-schema/item.json 

Exception: Could not read uri https://schemas.stacspec.org/v1.0.0-beta.1/item-spec/json-schema/item.json

This is a problem because schemas.stacspec.org only has schemas starting with 1.0.0-beta.2