Set Travis sudo: false, allegedly for speed
Clean up whitespace and fix a linting error

The "let (:key)" construct was the nearly-problematic one, because
association rules allow (:key) to be a "grouped expression".
Fix bogus description: sha1 => hmac
Add a tests that use the vault_digests feature

Although added for convenience, it was never exercised in tests in
c9ee3cd4ac0567a4e810ac377aac50bc1d0fe33c.