RUSTSEC-2019-0031: spin is no longer actively maintained about lucid

[CephalonRho]

We'll have to wait until the ring authors fix it in briansmith/ring#921.
We'll also have to update our dependencies to use a newer version of ring, which is probably going to involve changing parts of Lucid to use async-await syntax, although we can't do that without using the git repository revision of warp directly because version 0.2 isn't released on crates.io yet.

But this isn't too important to fix immediately, the spin crate still works and shouldn't break in the foreseeable future. Once everything is up-to-date and ring doesn't use it anymore this issue can be closed without requiring additional attention.

[thisisclint21]

Alright @CephalonRho!