jenkins is still using Ruby 1.8 when generating/testing JSDuck documentation. 1.8 was EOL'd last June (https://www.ruby-lang.org/en/news/2013/06/30/we-retire-1-8-7/) and the current version is 2.1.
JSDuck 4 runs perfectly well on newer versions, I think you could upgrade to at least 1.9 with no hiccups.
I noticed this when looking at a segfault that happened on this job:
– upgrading Ruby will probably fix this kind of problems too.