Rust-yaml dependency must be updated

Currently we use 0.4.2 (used by serde) and 0.3.5 (used by clap). Cargo audit is unhappy:

$cargo audit
    Fetching advisory database from `https://github.com/RustSec/advisory-db.git`
      Loaded 17 security advisories (from /home/ubuntu/.cargo/advisory-db)
    Scanning Cargo.lock for vulnerabilities (311 crate dependencies)
error: Vulnerable crates found!

ID:      RUSTSEC-2018-0006
Crate:   yaml-rust
Version: 0.3.5
Date:    2018-09-17
URL:     https://github.com/chyh1990/yaml-rust/pull/109
Title:   Uncontrolled recursion leads to abort in deserialization
Solution: upgrade to: >= 0.4.1

error: 1 vulnerability found!

I sent a PR against clap, opening this issue to track the update https://github.com/clap-rs/clap/pull/1396