Bump mdbook dependency to 0.4

[72636c]

This allows the preprocessor to parse a SUMMARY.md that contains book parts, a feature added in 0.4.0:

• https://github.com/rust-lang/mdBook/pull/1171
• https://github.com/rust-lang/mdBook/blob/master/CHANGELOG.md#mdbook-040

This seems to work locally without any code changes.

[72636c]

The cargo audit warning is a little out of our control:

├── mdbook v0.4.4
│   ├── tokio v0.2.24
│   │   ├── mio v0.6.21
│   │   │   ├── net2 v0.2.33

[boozook]

Thanks! ❤️