Updater should not send Kernel and GTK version
Not sure how exactly the script works that detects the .mar file the user needs to download but I guess the user should not be in a need to send the Kernel and GTK version to get the correct one. Testing a bit my update URL looks something like https://www.torproject.org/dist/torbrowser/update/alpha/Linux_x86-gcc3/Linux%203.14-2-686-pae%20(GTK%202.24.24)/4.0-alpha-2/en-US?force=1
I somehow doubt we have an own directory for every Kernel+GTK combination. Thus, what the updater should send in my opinion is something like
You have then the update channel, the platform + architecture, the current version and the language. That should be enough for a successful update.