Suppress htmlunit log messages in RobotFramework output

We need to find a way of how to configure robotframework/htmlunit to disable these logging messages.

See for details: https://github.com/MarkusBernhardt/robotframework-selenium2library-java/issues/85 and https://github.com/robotframework/MavenPlugin/issues/65