Update mdbook requirement from 0.3.1 to 0.4.9

[dependabot-preview[bot]]

Updates the requirements on mdbook to permit the latest version.

Release notes

Sourced from mdbook's releases.

v0.4.9

See https://github.com/rust-lang/mdBook/blob/master/CHANGELOG.md#mdbook-049 for a complete list of changes.

Changelog

Sourced from mdbook's changelog.

mdBook 0.4.9

7e01cf9...d325c60

Changed

• Updated all dependencies and raised the minimum Rust version to 1.42. #1528
• Added more detail to error message when a preprocessor fails. #1526
• Set max-width of HTML video tags to 100% to match img tags. #1542

Fixed

• Type errors when parsing book.toml are no longer ignored. #1539
• Better handling if mdbook serve fails to start the http server. #1555
• Fixed the path for edit-url-template if the book used a source directory other than src. #1554

mdBook 0.4.8

fcceee4...b592b10

Added

• Added the option output.html.edit-url-template which can be a URL which is linked on each page to direct the user to a site (such as GitHub) where the user can directly suggest an edit for the page they are currently reading. #1506

Changed

• Printed output now includes a page break between chapters. #1485

Fixed

• HTML, such as HTML comments, is now ignored if it appears above the title line in SUMMARY.md. #1437

mdBook 0.4.7

9a9eb01...c83bbd6

Changed

• Updated shlex parser to fix a minor parsing issue (used by the preprocessor/backend custom command config). #1471
• Enhanced text contrast of light theme to improve accessibility. #1470

Fixed

• Fixed some issues with fragment scrolling and linking.

... (truncated)

Commits

• 6425c29 Merge pull request #1562 from ehuss/bump-version
• d0bb830 Update to 0.4.9
• d325c60 Merge pull request #1554 from joshrotenberg/edit_url_custom_src
• e9e889f Merge pull request #1560 from joshrotenberg/clippy_path_fixes
• 05edc44 clippy: PathBuf to Path
• 22ea5fe Merge pull request #1557 from xrmx/cargo-clippy-fixes
• 714c5fb renderer: remove redundant clone in CmdRenderer
• 56ceb62 book: simplify is_draft_chapter
• c1b2bec book: use non_exhaustive attribute for struct Book
• 8201b41 Merge pull request #1555 from joshrotenberg/exit_serve_if_panic
• Additional commits viewable in compare view

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Superseded by #18.