Fix mage-sdk-cpp project path.

mage-sdk-cpp-cocos2d-x project refers adjacent project file like this.

> ls
./mage-sdk-cpp-cocos2d-x
./mage-sdk-cpp

mage-sdk-cpp-cocos2d-x project should refer to submodule project file.
Merge pull request #2 from tokutake/master

Fix mage-sdk-cpp project path.