x.y is not a valid version for toolchain install

[nagisa]

took me a while to figure out I need version in x.y.z format.

[brson]

As with some other features, this will require an index of all releases on the server to make work.

A related issue is that rustup doesn't ever do any fuzzy matching on semver - all versions are exact. This will become an important matter when Rust makes its first stable point release. There will probably be a reasonable expectation that rustup upgrades from X.Y.0 to X.Y.1 automatically.

[steveklabnik]

Also, it's worth noting that the semver spec is a bit murky around x.y being equivalent to x.y.z, so the semver crate doesn't parse it as a version. There's a PR for it, but I haven't decided if I want to merge it or not.

[Diggsey]

Dupe of #794