.travis.yml: disable HTTP replay
.travis.yml: disable HTTP replay

Don't use HTTP replay on Travis for now.

- The replay file includes headers that have the Go version. So we can't
  replay across versions (the headers don't match).

- Travis was giving "cannot assign requested address" errors.

The first of these is not hard to fix, but the second may be. Will revisit
at a later time.

Change-Id: I4fbed07ac66c7321c46181a97ef62ea7008539cf
Reviewed-on: https://code-review.googlesource.com/29870
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