Bump mdbook dep to pick up new 'create missing' toggle feature.

[frewsxcv]

This will avoid obscure Travis CI error messages:

• Add as_bytes() for FromUtf8Error. #40290 (comment)

Original mdbook issue:

• https://github.com/azerupi/mdBook/issues/253

mdbook PR:

• https://github.com/azerupi/mdBook/pull/254

[rust-highfive]

Thanks for the pull request, and welcome! The Rust team is excited to review your changes, and you should hear from @alexcrichton (or someone else) soon.

If any changes to this PR are deemed necessary, please add them as extra commits. This ensures that the reviewer can see what has changed since they last reviewed the code. Due to the way GitHub handles out-of-date commits, this should also make it reasonably obvious what issues have or haven't been addressed. Large or tricky changes may require several passes of review and changes.

Please see the contribution instructions for more information.

[frewsxcv]

r? @steveklabnik

[frewsxcv]

Hold off on r+'ing this, I want to confirm this is working as intended.

[frewsxcv]

Blocked on https://github.com/azerupi/mdBook/pull/259.

[steveklabnik]

@frewsxcv looks like that was merged and published

[frewsxcv]

cool, everything is working.

[00:36:22] An error occured:
[00:36:22] '/checkout/src/doc/unstable-book/src/foo.md' referenced from SUMMARY.md does not exist.
[00:36:22] 
[00:36:22] 
[00:36:22] command did not execute successfully: "/checkout/obj/build/x86_64-unknown-linux-gnu/stage0-tools/x86_64-unknown-linux-gnu/release/rustbook" "build" "/checkout/src/doc/unstable-book" "-d" "/checkout/obj/build/x86_64-unknown-linux-gnu/doc/unstable-book"
[00:36:22] expected success, got: exit code: 101
[00:36:22] 
[00:36:22] 
[00:36:22] Build completed unsuccessfully in 0:02:22
[00:36:22] make: *** [all] Error 1
[00:36:22] Makefile:24: recipe for target 'all' failed

@steveklabnik this is ready for review now

[frewsxcv]

Looks like the build was cancelled on Travis, but this should be still good to go.

[steveklabnik]

This looks good; I've restarted the build. r=me once travis passes.

[frewsxcv]

@bors r=steveklabnik

[bors]

📌 Commit a517a96 has been approved by steveklabnik