update tab not updated

[Stijn Husniaux]

When receiving the notification in AppCenter that there are updates are available, this is not updated when you update elementary by terminal commands. This is only done when you manually go to the tab in AppCenter after the system has been updates

This can be reproduced by:
- Update notification in AppCenter, number of updates is shown
- open terminal (of choice)
- run "sudo apt-get update && sudo apt-get dist-upgrade"
- wait until the update has finished
- look to the amount of updates available in AppCenter, this is the same as before the update.

Please note that the application AppCenter remains closed during the updates.