Fix tests

I noticed some failing tests so I had a go at fixing them.

1. The new version of [Retryable](https://rubygems.org/gems/retryable) has changed its interface and [no longer monkey patches Kernel](https://github.com/nfedyashev/retryable/commit/9dc4579a6ae7054ae1495bacd103c36fb0dd7889).
2. The MD5 hash of the chef-zero generated `metadata.json` from the test fixtures seemed to be wrong. I updated it to be the hash as generated by the latest 1.5.x release of chef-zero (1.5.6).