have default for major version

... as well as major/minor and latest.

library/jenkins

@@ -1,4 +1,5 @@
 # maintainer: Michael Neale <[email protected]> (@michaelneale)
 
 latest: git://github.com/cloudbees/jenkins-ci.org-docker@2627b96534028614f718315d7f84eb49362e445d
+1.554: git://github.com/cloudbees/jenkins-ci.org-docker@2627b96534028614f718315d7f84eb49362e445d
 1.554.3: git://github.com/cloudbees/jenkins-ci.org-docker@2627b96534028614f718315d7f84eb49362e445d