Remove support for alternate JSON backends

See #195; there wasn't broad agreement there, but if you look at the multi_json PR that @iainbeeston linked (https://github.com/intridea/multi_json/pull/113), you can see just how broadly this is happening across the ruby ecosystem right now. With the dropping of support for 1.8 (#205), we have a built-in, not-that-bad JSON library on all supported runtimes, so we should just use that, IMO.

This will have conflicts with #205, since both add a Changelog.md. I assume that one will land first, so I'll rebase later.
Rather than merge (merge commits?) I think it'd be good if we could rebase 3.0 on master as needed. Harder but cleaner