Jenkins security update

I've updated Jenkins from 1.606 to 1.625.2 today to get a fix for an
urgent security issue.

I hit a few issues while doing so, namely that the slaves now needed
Java 7 (I thought it was only the master) which I pushed out. Some also
then failed to checkout sources from GitHub with SSL issues, which was
resolved by updating SSL libs and certs packages.

They do all appear to be online again, but if you have any jobs that
fail for particularly odd reasons (checkout failures, Java exceptions
etc.) in the next day or two, please reply to let me know.

ยทยทยท -- Dominic Cleal