Use Browser/tbb_version.json file to detect installed version

[boklm]

Since #307, the version number of the installed version is read from the ChangeLog.txt file.

Tor Browser 8.0a4 is introducing a new tbb_version.json file containing the version number:

$ cat tor-browser_en-US/Browser/tbb_version.json 
{"version":"8.0a4","architecture":"linux64","channel":"alpha","locale":"en-US"}

When this file exists, torbrowser-launcher should use it instead of the ChangeLog.txt file.