Switch to clippy_utils on crates.io (#186)

`clippy_utils` used to be a Git dependency, but thanks to
rust-lang/rust-clippy#13556 it is now being
published on <https://crates.io>! This PR switches to the crates.io
version, and bumps the pinned nightly toolchain to match it (as
specified [here](https://lib.rs/crates/clippy_utils)).

We still want to pin a specific version of `clippy_utils`, since it does
not follow semantic versioning, but now we have the ability to publish
ourselves to crates.io!

Closes #150.