Remove multi_json dependency

As you are probably aware, [Ruby 1.8 will stop being updated—even for critical security issues—past June](http://www.ruby-lang.org/en/news/2011/10/06/plans-for-1-8-7/). This patch changes the minimum Ruby version to 1.9, as has [already been done in the forthcoming version of Rails](https://github.com/rails/rails/commit/e883c06a4f924cc4ba74efe4dad36394fad26fa0), currently in release candidate.

Since Ruby 1.9 includes `json` in the standard library, `multi_json` is no longer necessary. I am a longtime maintainer of `multi_json` but [will stop supporting the library in June](https://github.com/intridea/multi_json/pull/113#issuecomment-17668823).

This patch replaces `multi_json` with stdlib `json`. It will use the `json` gem if a newer version is available.

I have already submitted [a similar pull request to `execjs`](https://github.com/sstephenson/execjs/pull/126) and will shortly be submitting pull requests to `sprockets` and `rails` itself, so `multi_json` need not be a dependency of Rails 4 applications. This should have the effect of making JSON parsing and generation somewhat faster by removing an abstraction layer that is no longer necessary.