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.
Merge branch 'master' into use_jenkins_lts

Conflicts:
	hieradata/common.yaml
Merge pull request #5 from alphagov/use_jenkins_lts

Switch to Jenkins LTS releases