Warn if stdlib json is used.

MultiJson loads stdlib version of `json` gem by default. It's broken in many ways, doesn't support some of the options we pass there and segfaults on nil input (see https://github.com/flori/json/issues/165).

I think MultiJson should check the version of `json` gem loaded and complain loudly if the version is not the latest one.

What do you think, @sferik?