release upgrader doesn't update partner repo URL if it isn't enabled

[Alistair Buxton]

Steps to reproduce:

1. Have an existing install of an old release eg 13.10, where the partner repo has never been enabled.
2. Upgrade it to 14.04.
3. Enable to partner repo in the recommended way using Software Sources.

Now check /etc/apt/sources.list: It will point to the partner repo for saucy.

Impact:

If the user attempts to install Skype, they will get an out of date version (4.2) which just says "Skype cannot connect" repeatedly.

ProblemType: Bug
DistroRelease: Ubuntu 14.04
Package: ubuntu-release-upgrader-core 1:0.220.6
ProcVersionSignature: Ubuntu 3.13.0-44.73-generic 3.13.11-ckt12
Uname: Linux 3.13.0-44-generic x86_64
ApportVersion: 2.14.1-0ubuntu3.6
Architecture: amd64
CrashDB: ubuntu
Date: Fri Jan 23 20:21:21 2015
InstallationDate: Installed on 2013-12-22 (397 days ago)
InstallationMedia: Ubuntu 13.10 "Saucy Salamander" - Release amd64 (20131016.1)
PackageArchitecture: all
SourcePackage: ubuntu-release-upgrader
UpgradeStatus: Upgraded to trusty on 2014-06-20 (217 days ago)
VarLogDistupgradeTermlog: