Fixes compiler errors for unknown instance method "indexPath"

[nataliepo]

Just checked this project out on Xcode 7.3, and, a fresh build gives me two compiler errors:

    ASHSpringyCollectionViewFlowLayout.m:55:106: error: no known instance method for selector 'indexPath'
            BOOL currentlyVisible = [itemsIndexPathsInVisibleRectSet member:[[[behaviour items] firstObject] indexPath]] != nil;  

and

    ASHSpringyCollectionViewFlowLayout.m:61:76: error: no known instance method for selector 'indexPath'
            [self.visibleIndexPathsSet removeObject:[[[obj items] firstObject] indexPath]];

This branch declares the id<UIDynamicItem> object, and does some basic guarding in case the object isn't a UICollectionViewLayoutAttributes object, though in this project it always is. Compiles now and example matches gif on the README.