Build in regression test case retries

[dhermes]

Add in one retry for network failures to reduce (eliminate?) flaky / intermittent failures.

For example
https://travis-ci.org/GoogleCloudPlatform/gcloud-python/builds/46346592
after #509 failed on code that didn't even change in the PR.

[dhermes]

See https://github.com/dhermes/gcloud-python/tree/retry-flaky-regression-cases, will submit PR

[dhermes]

Another regression failure:
https://travis-ci.org/GoogleCloudPlatform/gcloud-python/builds/46219691

This one is actually a test failure (F) instead of an error (E) caused by a network 404 as above.