On CI only use mdbook 0.1.7.

Author: ehuss

.travis.yml

@@ -48,7 +48,7 @@ matrix:
            ALT=i686-unknown-linux-gnu
       rust: nightly
       install:
-        - mdbook --help || cargo install mdbook --force
+        - travis_retry curl -Lf https://github.com/rust-lang-nursery/mdBook/releases/download/v0.1.7/mdbook-v0.1.7-x86_64-unknown-linux-gnu.tar.gz | tar -xz --directory=$HOME/.cargo/bin
       script:
         - cargo test --features=deny-warnings
         - cargo doc --no-deps