Changed the source_file regex for CocosDenshion to work with the cocos2D 2.1 release

@jaisanth's [update](https://github.com/CocoaPods/Specs/commit/445ee661695f34d6f86b53d901662b5aec9bd7ef) of the cocos2D podspec seemed to break CocosDenshion support for me as the subspec source_files regex wasn't compatible with the directory structure of [release-2.1](https://github.com/cocos2d/cocos2d-iphone/tree/release-2.1/CocosDenshion). It seems that the subspec is using the directory structure of [Steve Oldmeadow's fork](https://github.com/steveoldmeadow/cocos2d-iphone/tree/develop/CocosDenshion)

This slight change should fix that and allow CocoaPods user to keep on using CocosDenshion.