Replaced cc.Reader with cc.BuilderReader

I've replaced calls to cc.Reader.loadAsScene with cc.BuilderReader.load as indicated by looking into the code in https://github.com/cocos2d/cocos2d-js-tests/pull/74/files#L3R43, as cc.Reader is undefined.

The JS API documentation for CocosBuilder doesn't contain reference to any of these methods, so it is unclear what exactly the parameters do. My "fix" is inferred from the previous mentioned code. Would be great to have the JS API docs reflect the cc.BuilderReader implementation, which seems to be the current method for loading a scene.