Update to Jenkins 1.554.2 (#395)

I'm not that tempted to also upgrade all the plugins we have installed at the same time. Best is to change as less as possible factors for regressions. At least I have checked that no plugins we have installed are marked as incompatible. So this update should be smoothly, and the good thing is that the Java security prompt will no longer show up when starting the Jenkins client on the slaves!

Please review.