remove subclass restriction on IGListCollectionViewLayout
remove subclass restriction on IGListCollectionViewLayout

Summary: remove subclass restriction on IGListCollectionViewLayout, so IGListCollectionViewLayout can be subclassed to support customized layout behavior.

Reviewed By: rnystrom

Differential Revision: D4700759

fbshipit-source-id: 1a22d1b981251ae60028724ae220e49d2d37c190

Instagram/IGListKit,zhubofei/IGListKit,Instagram/IGListKit,zhubofei/IGListKit,Instagram/IGListKit,zhubofei/IGListKit,zhubofei/IGListKit,Instagram/IGListKit,Instagram/IGListKit,zhubofei/IGListKit