Move from rust-toolchain to rustup overrides for rust version selection.

This is a complicated one. There are a few factors in action here:

* For background, the `maud` library requires nightly. The macro shenanigans it pulls are not supported on stable.

    <https://maud.lambda.xyz/getting-started.html>

* Travis installs Rust "cold," with no prior Rust installation, using `rustup`, for each entry in the build matrix. Travis _always_ uses the "minimal" profile.

    <https://docs.travis-ci.com/user/languages/rust/#choosing-a-rust-version>

* Since 2020-06-10, the "nightly" release of the Rust language has not included the `rustfmt` component.

    <http://rust-lang.github.io/rustup-components-history/>

    Note that the information on that page is likely to be different by the time you look at it.

* The Travis test suite requires `rustfmt` to validate commits. See `.travis.yml` for that one.

* Rustup will prefer `rust-toolchain` files over command-line options when selecting a Rust version to install.

Because of this wide range of factors, since the eleventh, it has not been possible to run `rustfmt` via `cargo fmt` on Travis. Tests have been broken since we added `rustfmt` as a mandatory step.

This change causes Travis to use both a known-good nightly (June 10th's), and also to try the build with current nightly. Current nightly is permitted to fail; once this starts passing, we can make that build mandatory and reinstate `rust-toolchain`.

Note that this doesn't affect most developers, as they use `rustup`'s default profile, which always includes `rustfmt`. `rustup` accomplishes this by walking back in time until it finds a nightly build that includes all the components in the profile, but it only does this when installing a fresh toolchain, not when trying to add a component to an installed toolchain.

I would vastly prefer to keep `rust-toolchain`, but it interferes with `rustup`'s installation logic, so it has to go for now.

Score one for never using `nightly`...

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