Bump mdbook: 0.1 -> 0.2.

Confirmed locally that 0.2 builds the book fine.

Author: frewsxcv

.travis.yml

@@ -7,7 +7,7 @@ before_install:
   - git --version
 script:
   - rev=$(git rev-parse --short HEAD)
-  - (cargo install mdbook --vers '^0.1.0' --force || true)
+  - (cargo install mdbook --vers '^0.2.0' --force || true)
   - ./generate-book.sh
   - cd book
   - git init