Fix all Cocos2D podspecs with versions <=2 by pointing to the Cocos2d git repository backup

The old cocos2d repository was changed and its history rewritten in order to reduce its size.(http://www.cocos2d-iphone.org/forums/topic/important-read-me-git-users/) 
Only the new 3.x tags were kept, so that makes installing cocos2d via CocoaPods impossible as the specs reference versions <=2.1

Fortunately a copy of the old cocos2d repository was kept at https://github.com/cocos2d-classic/cocos2d-iphone.

This PR modifies all the cocos2s pod specs to point to the cocos2d-classic repository.