Update mdbook version

This version of mdbook includes
https://github.com/azerupi/mdBook/pull/207 , which is needed so that
we can start doing linkchecker on the various books.

Author: steveklabnik

src/tools/rustbook/Cargo.toml

@@ -8,5 +8,5 @@ license = "MIT/Apache-2.0"
 clap = "2.19.3"
 
 [dependencies.mdbook]
-version = "0.0.14"
+version = "0.0.15"
 default-features = false