Update mbstring to support latest libmbfl

Summary: I am updating `libmbfl` to the latest code, bringing us from what was used in PHP 5.2 to
what is currently being used in PHP 5.6. We needed to modify `ext_mbstring.cpp` to
support this update.

Reviewed By: @markw65

Differential Revision: D1713697

Signature: t1:1713697:1418140057:42b2779814ce01d45ce5b53c5b17cbb0f7819b31