travis: login to Docker Hub
travis: login to Docker Hub

Signed-off-by: Nicolas Busseneau <418d940643b1975d62234ee01246ad4b58904184@isovalent.com>

tklauser/cilium,michi-covalent/cilium,tklauser/cilium,cilium/cilium,michi-covalent/cilium,michi-covalent/cilium,michi-covalent/cilium,cilium/cilium,tklauser/cilium,tklauser/cilium,cilium/cilium,cilium/cilium,michi-covalent/cilium,tklauser/cilium,cilium/cilium