Only install mdbook if building the book

Author: James Munns

.travis.yml

@@ -18,7 +18,7 @@ matrix:
 
 before_script:
   - rustup component add rustfmt
-  - (test -x $HOME/.cargo/bin/mdbook || ./ci/install-mdbook.sh)
+  - if  [[ -n "$BUILD_BOOK" ]]; then (test -x $HOME/.cargo/bin/mdbook || ./ci/install-mdbook.sh); fi
 
 script:
   - if ![[ -n "$BUILD_BOOK" ]]; then cargo check --all --benches --bins --examples --tests && cargo test --all; fi