Add a way to disable libravatar.

On connections where we can't hit libravatar for whatever reason, it
is nice to be able to run the tests without having them stall trying to
connect to libravatar.org. This gives us a way to disable it.

Signed-off-by: Ricky Elrod <ricky@elrod.me>
Merge pull request #52 from fedora-infra/libravatar-optional

Add a way to disable libravatar.