fix incompatible pointer type warning for Deployment Targets >= iOS 6.0

The _layout ivar is of type PSTCollectionViewLayout; if targeting >= iOS6.0, PSUICollectionViewFlowLayout is redefined to UICollectionViewFlowLayout, which is an incompatible pointer type with PSTCollectionViewLayout.

I believe the intent was to assign PSTCollectionViewFlowLayout, regardless of deployment target.