Deal with Stefan Birkner

[radek-macha]

Stefan Birkner's fishy testing library for exit code assertions broke the mvn clean install build recently. The problem is due to some SSL certificate invalidity on the OSS Sonatype repository, which is being used for pulling a SNAPSHOT version of the JUnit framework (which seems quite harmful to me).

If there's any chance of removing this library from task-api, I'd go for it - we definitely want to eliminate this kind of build instabilities, which means using standard Maven repositories if possible.