Fix JSON-LD 1.1 @version processing

[dlongley]

See: w3c/json-ld-syntax#99

[davidlehn]

Is the current JSON-LD 1.1 test conformance enough to close this?

[gkellogg]

Yes, I think the current test coverage is adequate. Note that most tests still assert @version, or do so through options, but there are tests that look for it without options, or with the version option set explicitly to json-ld-1.0.