Update mdbook to v0.4.5 (#286)

Author: milesfrain

.travis.yml

@@ -4,7 +4,7 @@ language: node_js
 node_js: 12
 install:
   - npm install -g purescript spago
-  - wget https://github.com/rust-lang/mdBook/releases/download/v0.3.7/mdbook-v0.3.7-x86_64-unknown-linux-gnu.tar.gz
+  - wget https://github.com/rust-lang/mdBook/releases/download/v0.4.5/mdbook-v0.4.5-x86_64-unknown-linux-gnu.tar.gz
   - tar -xzvf mdbook-*.tar.gz
 script:
   - travis_fold start "build"