Switch to using new Tor Project dist URL.

If I recall correctly, weasel (Tor Project's volunteer lead sysadmin)
tried to switch everything to using https://dist.torproject.org/ last
week, and then added an Apache redirect from
https://www.torproject.org/dist to https://dist.torproject.org after
discovering that some things were still trying to use the old URL. We
should switch to the new one, so that someday weasel can remove the
redirect.

 * CHANGE `default_mirror` to https://dist.torproject.org/