Upgrade obfs4proxy to fix meek's certificate pinning
obfs4proxy's meek-lite transport stopped working on 7 December due to a change in the TLS certificate chain of the Azure infrastructure. This will prevent Briar users in China from connecting to Tor, as well as any users in other countries who are on IPv6-only networks and have bridges enabled.
There hasn't been a new obfs4proxy release since the fix was merged, so we'll need to build from master. I suggest we use
0.0.12-dev-<commit ID> as the version number, as this isn't a snapshot build in the Maven sense (the artifact won't change when changes are made upstream).