mdBook: download ældre versioner

See https://github.com/rust-lang/mdBook/blob/master/CHANGELOG.md#mdbook-054 for a complete list of changes.

See https://github.com/rust-lang/mdBook/blob/master/CHANGELOG.md#mdbook-053 for a complete list of changes.

See https://github.com/rust-lang/mdBook/blob/master/CHANGELOG.md#mdbook-052 for a complete list of changes.

See https://github.com/rust-lang/mdBook/blob/master/CHANGELOG.md#mdbook-051 for a complete list of changes.

The 0.5.0 release is the next major release of mdBook, containing over 130 PRs since 0.4.52! The primary focus for this release has been an evolution of the Rust APIs to make it easier to maintain, to evolve in a backwards-compatible fashion, to clean up some things that have accumulated over time, and to significantly improve the performance and compile-times.
This release also includes many new features described in the CHANGELOG.
We have prepared a 0.5 Migration Guide to help existing authors switch from 0.4.

See https://github.com/rust-lang/mdBook/blob/master/CHANGELOG.md#mdbook-050-beta2 for a complete list of changes.

See https://github.com/rust-lang/mdBook/blob/master/CHANGELOG.md#mdbook-050-beta1 for a complete list of changes.

See https://github.com/rust-lang/mdBook/blob/master/CHANGELOG.md#mdbook-0452 for a complete list of changes.

See https://github.com/rust-lang/mdBook/blob/master/CHANGELOG.md#mdbook-0451 for a complete list of changes.

See https://github.com/rust-lang/mdBook/blob/master/CHANGELOG.md#mdbook-0448 for a complete list of changes.