Update Manager does not respect Package Manager distribution preference

[Gil Shwartz]

>> Version:

Linux Mint 17.2

>> Steps:

1. In the Package Manager distribution tab I select and set the "Prefer versions from" option to "trusty".
2. New Firefox version comes out for Trusty.
3. Update Manager advise to update Firefox to the new Trusty version, which I do. Update succeeds.
4. New Firefox version (actually the same version as Trust, e.g. right now 41.0.2) comes out for Rafaela.
5. Update Manager advise to update Firefox to the new Rafaela version, which I do. New version is downloaded, but fail to install, most probably due to the Package Manager preference in 1.
6. Update Manager closes following the "update", but keeps informing of a new update to package Firefox (basically an infinite update loop).

>> Expectations:

A. In sync with the Package Manager preference, Update Manager shall not report of an available Rafaela update.

optionally,

B. If Update Manager does report about an update (i.e. new Rafaela update) it should succeed in installing it.

>> Workaround:

The preference in Package Manager was changed to prefer Rafaela. Following the change, the update manager installed the Rafaela Firefox successfully.