travis: Remove image push to docker hub
travis: Remove image push to docker hub

We don't really need to push every travais image build for PR on
docker hub.
There's also an issue because we're using docker login with
credentials that can't be used for everybody.

Signed-off-by: Dimitri Savineau <a50a620e679eed14f4e99998b821cea8884cf228@redhat.com>

ceph/ceph-docker,ceph/ceph-docker