Send service_name and service_job_id only if both are set

[mrc0mmand]

By default we set service_name to travis-ci if service_name is unset, but that in combination with an empty service_job_id causes all such submitted results to end up in a single build (as they have the same job ID) instead of the expected behavior where each submitted result has a separate build.

The API[0] states that we should either use repo token or both service job ID and service name, so let's do just that.

[0] https://docs.coveralls.io/api-introduction