Testing for late-arriving facts

Include in package integration tests [`event_update`](integration_tests/data/snowplow/event_update.csv):
* new event, same `session_id` as earlier event
* new event, same `page_view_id` as earlier event (?)
* same `event_id` as earlier event (?)

Our approach to these edge cases has been in flux over the past few releases. I think we have a good grasp of what desired behavior is as of [0.7.1](https://github.com/fishtown-analytics/snowplow/tree/0.7.1). Let's include these cases in our integration tests to ensure the modeling operates as expected.