fix for incorrect image cache directory path

- When using multiple cache instances of LRImageCache the path returned from LRPathToImageCacheDirectory both returns the path for the first cache created. The dispatch_once looks like it would work fine if  LRPathToImageCacheDirectory was an instance method instead of an NS_INLINE.
Hey @seanroehnelt. Thanks for this. As you said, I think it would be a better fit to have this method as an instance one. Would you change it?