hyper-spec does not require json

newer Opal versions do not automatically include the json library, which hyper_spec depends on (depends on Object#to_json)

In most cases it doesn't matter since the json library will get included by some other dependency (i.e. opal-browser, opal-jquery, and most of the hyperstack gems)

There is however a chance that you could be using hyper-spec with a slimmed down opal App and get an error saying to_json aint there.

For now we can document that you have to require to_json (at least for test) but it would be nice to learn how to do this from within opal sprockets so it was automagic.