Upgrade signatory -> 0.15; yubihsm -> 0.28; use chacha20poly1305 crate

This commit updates `signatory` and `yubihsm` to the latest versions.

This requires either upgrading *ring* to 0.16, or replacing it:

informalsystems/tendermint-rs#11

This commit has opted to replace it with the (i.e. my)
`chacha20poly1305` crate:

https://github.com/RustCrypto/AEADs/tree/master/chacha20poly1305

While *ring* is a great crate with excellent cryptographic
implementations, the inability to link two versions of `ring-asm` is a
major impediment to doing upgrades, as it requires updating all
*ring*-dependent dependencies at once, or not at all, which so far has
been a major blocker and has been stalling KMS development for awhile.
As we're only using it for ChaCha20Poly1305, it seems like a rather
heavyweight dependency just to serve that one purpose.

.circleci/config.yml

@@ -49,14 +49,11 @@ jobs:
           rustc --version
           cargo --version
           cargo test --all-features -- --test-threads 1
-    # TODO(tarcieri): upgrade cargo-audit and/or yubihsm-rs
-    # Audit is failing on RUSTSEC-2016-0004 and cargo-audit is too old to
-    # support the --ignore option
-    #- run:
-    #    name: audit
-    #    command: |
-    #      cargo audit --version
-    #      cargo audit
+    - run:
+        name: audit
+        command: |
+          cargo audit --version
+          cargo audit
     - run:
         name: validate against test harness
         command: |