Bump mdbook (rust-lang#321)

* Bump mdbook

* Run `cargo update -p mdbook`

Author: tesuji

.gitignore

@@ -1,2 +1,3 @@
-book
-.blacksmith-cache.json
+/.blacksmith-cache.json
+/book
+/target/

.travis.yml

@@ -1,6 +1,10 @@
 language: rust
 dist: xenial
 
+branches:
+  only:
+    - master
+
 env:
   RUSTINFRA_DEPLOY_DIR: book
   RUSTINFRA_CLOUDFRONT_DISTRIBUTION: E12A3GKHZSREHP
@@ -11,15 +15,17 @@ cache:
   directories:
     # cache: cargo
     - $HOME/.cargo
-    - $TRAVIS_BUILD_DIR/target
+    - target
+before_cache:
+  - cargo install --debug cargo-cache
+  - find $HOME/.cargo/bin/ ! -type d -exec strip {} \;
+  - cargo cache --autoclean
 
 before_script:
-  # TODO: Installing mdbook from crates.io takes quite a long time. This could
-  # be reduced by downloading the release from GitHub directly.
-  - mdbook -V || cargo install mdbook --vers '=0.3.1'
+  - export CARGO_TARGET_DIR=$(pwd)/target
+  # NOTE: To increase caching efficiency, keep the same mdbook version
+  # in blacksmith/Cargo.lock too
+  - cargo install -v --debug --locked mdbook --vers '=0.3.6'
 script:
   - mdbook build
   - cp website_config.json book/
-branches:
-  only:
-    - master