Remove multi_json gem, rely on Ruby 1.9 JSON

MultiJson isn't all its cracked up to be. It can introduce slowness, its been removed as a dependency in activesupport, and there's now full, fast support for JSON in Ruby 1.9+. More info: https://github.com/intridea/multi_json/pull/113

The fewer dependencies Crepe relies on, the better. If you need super-duper fast JSON in your app, use the YAJL gem and simply `require 'yajl/json_gem'` for full compatibility (though Ruby's JSON is now faster then the YAJL readme declares).