- switch to "project authors" style copyright. Instead of an AUTHORS
file (https://opensource.google/docs/releasing/authors/), I just list
Google as a major copyright holder in the README.
- use SPDX style license headers in source files
- remove CLA requirement from contributing docs
This is a bit cleaner than the gcs cache that was vendored in, is
properly licensed, and uses Google's application default credentials,
which just magically works when run from AppEngine and GCE.