There was an issue with the YAML parser for v0.4.2 which rendered the information provided by the CHANGELOG and the announcement blog post inaccurate. When v0.4.2 was released, it was mentioned that the YAML parser engine had been switched from Syck to the now Ruby default Psych. Unfortunately, there remained a piece of code in Arachni’s RPC library that set the YAML engine to Syck, which went unnoticed.
In short, the YAML engine didn’t change, everything is business as usual. However, Psych will be the engine used in future releases and the experimental branch from now on.
Please forgive this oversight and I hope that it didn’t cause you any trouble.
PS. I’ll now update the CHANGELOG and release announcement to reflect the real state of affairs.