Update ruby-msgpack to 0.5.11.

2015-02-04 version 0.5.11:

* Fixed Encoding::CompatibilityError error when serializing a long string
  longer than write_reference_threshold (5KB by default) and whose encoding
  is not ASCII-8BIT.
* Fix read_(map|array)_header (@funny-falcon++).
* Internally, rename msgpack_unpacker_{init,reset,destroy} functions
  so that we can load msgpack-c in the same process (@miihael++)

(taca)