ci: ignore background ping errors
ci: ignore background ping errors

We attempt to kill background ping process twice, and sometimes the
second kill fails, e.g.
https://travis-ci.org/hashicorp/nomad/jobs/486050357, let's not fail the job.

hashicorp/nomad,hashicorp/nomad,burdandrei/nomad,hashicorp/nomad,dvusboy/nomad,dvusboy/nomad,burdandrei/nomad,hashicorp/nomad,hashicorp/nomad,burdandrei/nomad,burdandrei/nomad,dvusboy/nomad,burdandrei/nomad,dvusboy/nomad,dvusboy/nomad,burdandrei/nomad,dvusboy/nomad,dvusboy/nomad,hashicorp/nomad,burdandrei/nomad