Use the long term support Tor release for bandwidth authorities

By the time we updated the bandwidth authority instructions to use 0.2.8, the stable release was 0.2.9. And it's now 0.3.0.

Ideally, we want to use the Tor release in Tor Browser stable (that's what most of our users are using).

But the latest stable long term support Tor release is easier to work out, and ~~~they track each other pretty closely~~ it doesn't change very often.