Rollup merge of #41572 - frewsxcv:bump-mdbook, r=steveklabnik

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

This will avoid obscure Travis CI error messages:

* https://github.com/rust-lang/rust/pull/40290#issuecomment-294137045

Original mdbook issue:

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

mdbook PR:

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

diff --cc src/Cargo.lock
index 0a584ac6b01c44101e2cbb33b0720d228f37f073,a1e403c6f1389c92c132e46c54603ef07594248f..659feef80a4b9a276d7554a01b92089733c82063
--- 1/src/Cargo.lock
--- 2/src/Cargo.lock
+++ b/src/Cargo.lock
@@@ -1001,10 -1001,10 +1001,10 @@@ source = "registry+https://github.com/r
  "checksum lazy_static 0.2.5 (registry+https://github.com/rust-lang/crates.io-index)" = "4732c563b9a21a406565c4747daa7b46742f082911ae4753f390dc9ec7ee1a97"
  "checksum libc 0.2.21 (registry+https://github.com/rust-lang/crates.io-index)" = "88ee81885f9f04bff991e306fea7c1c60a5f0f9e409e99f6b40e3311a3363135"
  "checksum log 0.3.7 (registry+https://github.com/rust-lang/crates.io-index)" = "5141eca02775a762cc6cd564d8d2c50f67c0ea3a372cbf1c51592b3e029e10ad"
- "checksum mdbook 0.0.19 (registry+https://github.com/rust-lang/crates.io-index)" = "2598843aeda0c5bb2e8e4d714564f1c3fc40f7844157e34563bf96ae3866b56e"
+ "checksum mdbook 0.0.21 (registry+https://github.com/rust-lang/crates.io-index)" = "f1e2e9d848514dcfad4195788d0d42ae5153a477c191d75d5b84fab10f222fbd"
  "checksum memchr 1.0.1 (registry+https://github.com/rust-lang/crates.io-index)" = "1dbccc0e46f1ea47b9f17e6d67c5a96bd27030519c519c9c91327e31275a47b4"
  "checksum num-traits 0.1.37 (registry+https://github.com/rust-lang/crates.io-index)" = "e1cbfa3781f3fe73dc05321bed52a06d2d491eaa764c52335cf4399f046ece99"
 -"checksum num_cpus 0.2.13 (registry+https://github.com/rust-lang/crates.io-index)" = "cee7e88156f3f9e19bdd598f8d6c9db7bf4078f99f8381f43a55b09648d1a6e3"
 +"checksum num_cpus 1.4.0 (registry+https://github.com/rust-lang/crates.io-index)" = "ca313f1862c7ec3e0dfe8ace9fa91b1d9cb5c84ace3d00f5ec4216238e93c167"
  "checksum open 1.2.0 (registry+https://github.com/rust-lang/crates.io-index)" = "3478ed1686bd1300c8a981a940abc92b06fac9cbef747f4c668d4e032ff7b842"
  "checksum pest 0.3.3 (registry+https://github.com/rust-lang/crates.io-index)" = "0a6dda33d67c26f0aac90d324ab2eb7239c819fc7b2552fe9faa4fe88441edc8"
  "checksum pulldown-cmark 0.0.14 (registry+https://github.com/rust-lang/crates.io-index)" = "d9ab1e588ef8efd702c7ed9d2bd774db5e6f4d878bb5a1a9f371828fbdff6973"