airbrake-ruby: always return `nil` on `#close`
airbrake-ruby: always return `nil` on `#close`

Although we explicitly say we return `void`, the type of the return value
changes from time to time (as we introduce new modifications to the method).

In order to be consistent, let's just return `nil` all the time.

airbrake/airbrake-ruby,airbrake/airbrake-ruby