42711 multiple log messages

- Bump Cocoalumberjack to version 2.0.0-rc
- Set up test logging inside dispatch_once block
To me this would be a lot simpler if you just called [removeAllLoggers](http://cocoadocs.org/docsets/CocoaLumberjack/1.1/Classes/DDLog.html#//api/name/removeAllLoggers) rather than using `dispatch_once`. `removeAllLoggers` offers a simpler reasoning than this does, because ensures a consistent logging state for all tests. It also would allow tests to add extra loggers if they wanted (e.g., to count log messages of a particular type) without intruding on other tests.