Change Cocos2d-x version to 3.1-alpha2
Change Cocos2d-x version to 3.1-alpha2

SIROK/growthpush-cocos2dx3,SIROK/growthpush-cocos2dx3,SIROK-archive/growthpush-cocos2dx3,SIROK-archive/growthpush-cocos2dx3,SIROK/growthpush-cocos2dx3,SIROK/growthpush-cocos2dx3,SIROK-archive/growthpush-cocos2dx3,SIROK-archive/growthpush-cocos2dx3