[Merge] ~aleasto/update-notifier:invoke-wayland into update-notifier:master

[Alessandro Astone]

> I'm not sure which active code path still uses invoke()

Ah, that would be when the systray icon is shown for "The update information is outdated. This may be caused by network problems [...]" 
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https://code.launchpad.net/~aleasto/update-notifier/+git/update-notifier/+merge/475790
Your team Ubuntu Core Development Team is requested to review the proposed merge of ~aleasto/update-notifier:invoke-wayland into update-notifier:master.