While the base URL remains configurable, this patch reverts back to using
Gravatar.

We noticed high latency with libravatar and contacted them. They are in the
process of migrarting to a better infrastructure (it's a single personal server
at the moment) so we'll re-evaluate once that has happened.

As for why not leave the default and change it on the meet.jit.si installation,
we don't want to kill their server :-)