Bug 1066989 - Fix the treestatus and clobberer URLs for Thunderbird

Now that bug 1035222 has corrected the base repo name, the clobberer
workaround added by bug 1035220 must be removed - and the treestatus
name generation tweaked.
Merge pull request #179 from mozilla/fix-thunderbird-urls

Bug 1066989 - Fix the treestatus and clobberer URLs for Thunderbird