STAC v1.1.0 validation fails due to missing `stac-spec/v1.1.0` directory

[pjhartzell]

Getting a FileNotFoundError after installing pystac from the latest SHA with the validation optional dependency. I believe the include entry in the MANIFEST.in file needs to be updated from:

pystac/validation/jsonschemas/stac-spec/v1.0.0/*.json

to:

pystac/validation/jsonschemas/stac-spec/v1.1.0/*.json

@gadomski I believe this was missed in #1427