update mdbook

Author: AndyGauge

README.md

@@ -19,7 +19,7 @@ If you'd like to read it locally:
 ```bash
 $ git clone https://github.com/rust-lang-nursery/rust-cookbook
 $ cd rust-cookbook
-$ cargo install mdbook --vers "0.3.5"
+$ cargo install mdbook --vers "0.4.5"
 $ mdbook serve --open
 ```
 

ci/install_deps.sh

@@ -18,7 +18,7 @@ if [[ "${CONTENT_TESTS:-}" == 1 ]]; then
         pyenv local 3.6.0
         pip3 install --user link-checker==0.1.0
     fi
-    cargo install mdbook --vers '0.3.5' --debug
+    cargo install mdbook --vers '0.4.5' --debug
 fi
 
 exit 0