httpreplay: use IPv4 sockets.
httpreplay: use IPv4 sockets.

Travis has a problem with IPv6 loopback: travis-ci/travis-ci#8711.

Change-Id: I4dc8160a7455e6f54a1c51f667c7f9159fba7e5b
Reviewed-on: https://code-review.googlesource.com/29892
Reviewed-by: kokoro <2ac7b1f3fa578934c95181d4272be0d3bca00121@google.com>
Reviewed-by: Jean de Klerk <605b93664484fed1e671a07bff68da39e73d0174@google.com>

googleapis/google-cloud-go,googleapis/google-cloud-go,googleapis/google-cloud-go,googleapis/google-cloud-go