upstart shouldn't depend on libjson0. libjson0 is a transitional package that can be safely removed once no package depends on it. libjson0 has only one dependency, libjson-c2, which upstart already depends on directly.
You should be able to run 'apt-get purge libjson0' without removing upstart.
ProblemType: Bug
DistroRelease: Ubuntu 14.04
Package: upstart 1.12.1-0ubuntu4.2
ProcVersionSignature: Ubuntu 3.13.0-37.64-generic 3.13.11.7
Uname: Linux 3.13.0-37-generic x86_64
ApportVersion: 2.14.1-0ubuntu3.5
Architecture: amd64
CurrentDesktop: Unity
Date: Thu Oct 23 15:21:08 2014
EcryptfsInUse: Yes
InstallationDate: Installed on 2012-11-01 (721 days ago)
InstallationMedia: Ubuntu 12.10 "Quantal Quetzal" - Release amd64 (20121017.5)
SourcePackage: upstart
UpgradeStatus: Upgraded to trusty on 2014-09-04 (49 days ago)
UpstartBugCategory: Session
UpstartRunningSessionCount: 1
UpstartRunningSessionVersion: init (upstart 1.12.1)
UpstartRunningSystemVersion: init (upstart 1.12.1)
This bug was fixed in the package upstart - 1.13.2-0ubuntu16
---------------
upstart (1.13.2-0ubuntu16) wily; urgency=medium
* Move from libjson0-dev to libjson-c-dev. (LP: #1384886)
-- Robert Ancell <email address hidden> Fri, 18 Sep 2015 06:29:15 +0200