upgrade msgpack 0.6 to 0.7 to use new jackson based api

Using Jackson `ObjectMapper` is easier to work with than the custom API found in 0.6. An example use case is a custom parser we use that converts MessagePack to JSON. Our MessagePack messages have values of different types (strings, floats, booleans, etc.). Jackson automatically handles converting these to the appropriate JSON values, but the 0.6 API does not provide any such utility and implementing it is non-trivial.