Dep updates

[tjkirch]

commit 413b93c010013d3bb0b9ab242d3886b37990a3e9
Author: Tom Kirchner <tjk@amazon.com>
Date:   Mon Mar 2 10:14:39 2020 -0800

Update webpki-roots-shim to match new versioned required by reqwest

commit d802422c98dc53974f9e2ad1e6138e601a155841
Author: Tom Kirchner <tjk@amazon.com>
Date:   Fri Feb 28 15:34:22 2020 -0800

Update Rust dependencies

This is a `cargo update`, followed by updating dependencies found through
`cargo outdated -w` and removing unused dependencies found by
`cargo +nightly udeps --workspace`.

The manual updates are:
* nix to 0.17; no important changes
* http to 0.2; no important changes
* reqwest to 0.10; we now use the 'blocking' module since we only use reqwest synchronously
* gptman to 0.6.1; no important changes

---

Testing done:

Made an AMI, it joined my cluster, a pod ran OK, system is running, API looks OK.

[tjkirch]

Rebase on develop, to get #805 so dependencies of removed code don't come into play.

Removed the commit that was erroneously trying to update cargo-deny config. Going to add a commit updating webpki-roots-shim next, and then update testing.

[tjkirch]

Added a commit that updates webpki-roots-shim so it properly replaces webpki-roots 0.18, which reqwest now depends upon. I did this with iliana to confirm the Cargo.lock was updated correctly. Working on updated testing now.

[tjkirch]

Updated testing; things seem good.