Disable CI email completely since configuration is not flexible

We can't let email configuration be the default since it would spam most
people in the GitHub org. However, we set it to a couple of addresses
explicitly, we get emailed with failures from other people's forks who
have set up CI for themselves.
Skip caching dependencies if Amazon secret is unavailable

Fixes CI in forked environment and enables usage of `cached-bundle`
script to just restore caches from S3 but never attempt to write them.
Merge branch 'forked-ci'