The ranking endpoints return a value for every day. This is rare, but it looks like sometimes this can be `null` (for example if you call http://bestgems.org/api/v1/gems/rack/daily_ranking.json the rank was `null` on `2013-07-02` ) and if the rank has _ever_ been `null` for a package in the past, the schema will fail validating the response.
This modifies the schema to allow a `null` value and adds a case to handle if the rank is `null` today.
closes#2647
7d1a6dd627