Switch to Jenkins LTS releases

These should be a bit more stable (better tested and changing less
frequently) than the weekly point releases.

Prompted by compatibility issues with plugins after an automatic upgrade on
`ci.alphagov` this week: gds/puppet@e7f69a2.

This will replace the existing repo entry but any installs on existing
machines will need to be downgraded manually. Might need testing with the
plugins you're currently playing with.