Final code for second code review for week 2

There is nothing wrong with this implementation, but I wanted to show you another way. Really I wanted to introduce you to the `try` method, in case you haven't seen it before. The contents of this method could be condensed down to this one line:

```ruby
Review.where(user_id: user.id, video_id: video.id).first.try(:rating)
```

`try` calls the method named with the symbol, but doesn't blow up if there is an error. This is great for times when it is calling a method on `Nil`.