Rollup merge of #40426 - steveklabnik:update-mdbook, r=alexcrichton

Update mdbook to new version

This brings back playpen integration for the books.

It'd be nice to land this before beta, but realistically that's probably not going to happen...

Author: frewsxcv

src/Cargo.lock

@@ -8,5 +8,5 @@ license = "MIT/Apache-2.0"
 clap = "2.19.3"
 
 [dependencies.mdbook]
-version = "0.0.17"
+version = "0.0.18"
 default-features = false