[coredata] Update to beta 6 (#2528)

This seems to only revert some nullability changes from beta 5.

Author: spouliot

src/coredata.cs

@@ -1730,12 +1730,10 @@ interface NSPersistentStoreDescription : NSCopying
 	interface NSPersistentContainer
 	{
 		[Static]
-		[return: NullAllowed]
 		[Export ("persistentContainerWithName:")]
 		NSPersistentContainer GetPersistentContainer (string name);
 
 		[Static]
-		[return: NullAllowed]
 		[Export ("persistentContainerWithName:managedObjectModel:")]
 		NSPersistentContainer GetPersistentContainer (string name, NSManagedObjectModel model);