Add minumum supported Rust version to cargo.toml

[tublitzed]

Looks like it's now supported.

Probably makes sense to do this to avoid future issues with versions that are too old.

[pjenvey]

Reopening because I just noticed the RFC hasn't been implemented yet: rust-lang/rust#65262

So #475 emits a harmless warning:

$ cargo build
warning: unused manifest key: package.rust

Let's revert it and just add the minimum version to the README for now

[tublitzed]

Ah shoot, thanks @pjenvey! Closing this now and will re-open if/when implemented.