Merge pull request #20 from ehuss/update-mdbook

Update mdbook

Author: taiki-e

.travis.yml

@@ -13,9 +13,7 @@ rust:
   - stable
 
 before_script:
-  - (test -x $HOME/.cargo/bin/cargo-install-update || cargo install cargo-update)
-  - (test -x $HOME/.cargo/bin/mdbook || cargo install --vers "^0.3" mdbook)
-  - cargo install-update -a
+  - cargo install --vers "^0.4.5" mdbook
 
 script:
   - mdbook build && mdbook test