consider using a GitHub mirror for tor-rust-dependencies

During a recent IP address change of the host of git.torproject.org (legacy/trac#33730 (moved)), a few Travis CI builds errored due to the unreachability of that host. (I think there were around five errored pull request builds? I restarted about three of them, ignoring the builds that were obsolete due to more recent changes.) I think this has happened before due to transient network faults.

We should consider making a GitHub mirror of the tor-rust-dependencies repository, and use that instead of the more canonical git.torproject.org location during Travis builds. (I'm guessing that the github.com repositories are very likely reachable if a Travis build starts successfully.) We might lose the ability to use the default git submodule setup done by Travis, though.

$ git submodule update --init --recursive
Submodule 'src/ext/rust' (https://git.torproject.org/tor-rust-dependencies) registered for path 'src/ext/rust'
Cloning into '/home/travis/build/torproject/tor/src/ext/rust'...
fatal: unable to access 'https://git.torproject.org/tor-rust-dependencies/': Failed to connect to git.torproject.org port 443: No route to host
fatal: clone of 'https://git.torproject.org/tor-rust-dependencies' into submodule path '/home/travis/build/torproject/tor/src/ext/rust' failed
Failed to clone 'src/ext/rust'. Retry scheduled
Cloning into '/home/travis/build/torproject/tor/src/ext/rust'...
fatal: unable to access 'https://git.torproject.org/tor-rust-dependencies/': Failed to connect to git.torproject.org port 443: No route to host
fatal: clone of 'https://git.torproject.org/tor-rust-dependencies' into submodule path '/home/travis/build/torproject/tor/src/ext/rust' failed
Failed to clone 'src/ext/rust' a second time, aborting