Jenkins LTS based official image

As discussed with Brian Goff - here is the official Jenkins repo for inclusion. 

Jenkins has a weekly non long-term-support release schedule - but we thought for the purposes of stackbrew we should stick to the LTS versions of Jenkins - which are infrequent enough that a pull request is feasible. 

For a logo, here is the official one in png form: https://camo.githubusercontent.com/73aa2e5efdce9c86a20922bc04f9c2bde852be4f/687474703a2f2f6a656e6b696e732d63692e6f72672f73697465732f64656661756c742f66696c65732f6a656e6b696e735f6c6f676f2e706e67 
good question @tianon. 

Firstly - note this is LTS release only. The non LTS version is weekly and seems too frequent for stackbrew (in the weekly case - it is a simple incrementing number). 

For LTS, we can see a list of them here: http://mirrors.jenkins-ci.org/war-stable/
So there is major/minor - but only the 2 levels. 
I will change this to have 1.554 as well as 1.554.3 - then when a LTS is released, say 1.554.4 - both latest and 1.554 will really point to 1.554.4 - while users can still choose an older 1.553.3 if they directly reference it. 

the minor releases are backports of critical fixes of course. 

This is similar to python - as you wisely point out - although only 2 levels deep, not 3.