Cannot bootstrap over IPv6

Using the current public TorBrowser build, I cannot connect to the Tor network from some machines.

torbrowser error dialog:

Tor failed to establish a Tor network connection.

Connecting to a relay directory failed (no route to host - 199.254.238.52:443).

torbrowser log:

18/10/2015, 21:46:46.600 [NOTICE] Opening Socks listener on 127.0.0.1:9150 

18/10/2015, 21:46:46.600 [NOTICE] Renaming old configuration file to "/Applications/TorBrowser.app/TorBrowser/Data/Tor/torrc.orig.1" 

18/10/2015, 21:46:46.600 [NOTICE] Bootstrapped 5%: Connecting to directory server 

18/10/2015, 21:46:46.600 [WARN] Problem bootstrapping. Stuck at 5%: Connecting to directory server. (Network is unreachable; NOROUTE; count 1; recommendation warn; host 7EA6EAD6FD83083C538F44038BBFA077587DD755 at 194.109.206.212:443) 

18/10/2015, 21:50:13.000 [WARN] Problem bootstrapping. Stuck at 5%: Connecting to directory server. (Network is unreachable; NOROUTE; count 2; recommendation warn; host 847B1F850344D7876491A54892F904934E4EB85D at 86.59.21.38:443) 

18/10/2015, 21:50:13.000 [WARN] Problem bootstrapping. Stuck at 5%: Connecting to directory server. (Network is unreachable; NOROUTE; count 3; recommendation warn; host F2044413DAC2E02E3D6BCF4735A19BCA1DE97281 at 131.188.40.189:443)

Two of the addresses attempted here actually belong to hosts that have IPv6 connectivity: https://gist.github.com/sdstrowes/9e5cf5a3f2fe48ef3979 These hosts have the correct ports open and are listening, but TorBrowser doesn't try to connect. It's not always the case that hosts at this stage have IPv6, but at least some do.

References to 127.0.0.0/8 smells v4-specific, which isn't useful without IPv4 end-to-end. NAT64 won't help in this instance.

Trac:
Username: sdstrowes