Remove multi_json dependency

As you are probably aware, [Ruby 1.8 will stop being updates—even for critical security issues—past June.](http://www.ruby-lang.org/en/news/2011/10/06/plans-for-1-8-7/) This patch make Ruby 1.9 the minimum version. This change has [already been made 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 will shortly be submitting similar pull requests to `sprockets`, `uglifier`, 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.