Updated Box2D to v2.3.1 and fixed unzip.h.

Version 2.3.1 is the newest tagged stable Box2D version in their source repository. I added a default parameter to Apply_ methods to keep compatibility.
Also fixed a compilation issue in unzip.h which breaks compilation against cocos2d-x v3.3.
This is my first contribution to this project, so I hope everything is with accordance to any guidelines etc. ;)