Check how we are using json-ld contexts

[cmungall]

See discussion on obo-tools list

also
json-ld/json-ld.org#714

don't have time to summarize just now. a change in the json-ld contexts spec breaks our use of contexts for OBO prefixes. robot iohelper code still seems to work but it's not clear if that will change with some future use of the json-ld library, or if we use in such a limited way we are protected. I think @balhoff did some preliminary analysis here.

we should probably change the docs here
http://robot.obolibrary.org/global

to say that we use using json-ld-like files...