Update travis build to use 2.X

Author: Alexis Hunt

.travis.yml

@@ -4,7 +4,7 @@ rust:
   - nightly
 
 install:
-  - 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=$HOME/.cargo/bin
+  - travis_retry curl -Lf https://github.com/rust-lang-nursery/mdBook/releases/download/v0.2.3/mdbook-v0.2.3-x86_64-unknown-linux-gnu.tar.gz | tar -xz --directory=$HOME/.cargo/bin
 
 script:
   - export PATH=$PATH:/home/travis/.cargo/bin && mdbook test