Raise exception if none of json libraries could be required.

[spacewander]

Instead of returning nil and let it break in another place, raise exception as soon as thing goes wrong.

[technoweenie]

Thanks, this seems helpful. May I make a suggestion about the code though? I think the message should mention the various gems that Sawyer works with too.

yajl || multi_json || json || begin
  raise RuntimeError, "Sawyer requires a JSON gem: yajl, multi_json, or json"
end

[spacewander]

@technoweenie LGTM.