Update BZObjectStoreRelationshipModel.h

Updated "attributeSequence" property definition from "assign" to "strong", due to a leak while registering classes to BZObjectStore. That leak leads to a EXC_BAD_ACCESS while trying to access the "attributeSequence" property via [NSObject valueForKey:] inside methods [BZObjectStoreClazzNSNumber storeValuesWithValue:attribute:]