Tor browser always displays the upgrade notification with Torbutton >= 1.6.8.0

Originally created by @anonym on #7097 (Redmine)

In our Tor browser builds, the pref torbrowser.version is set to Tails, and this does not work well together with upstream’s commit b9fb03cd (in Torbutton’s Git repo).

My guess is that there’s an assumption that torbrowser.version must look like an actual TBB version, like 3.5.4-Linux or whatever. With it set to something like Tails the upgrade notification is always blinking, suggesting that there’s an upgrade available.

Is this an upstream bug, or do we have to set torbrowser.version to something else from now on?

Feature Branch: feature/torbutton-1.6.9.0