Minor fix to in-app tracking timestamps

[NSDate timeSinceReferenceDate] does not produce a standard unix timestamp and was being used in an in-app notifications tracking call. By changing it to [NSDate date], the mixpanel library is able to handle the conversion from an NSDate object to the appropriate timestamp further down the line.

This tracking call only affects a planned dashboard for in-app notifications and will not cause any noticeable or functional changes to apps regardless of whether or not they are using the feature.