Merge pull request #181 from ehuss/install-upgrade

Try cargo install-upgrade for mdbook.

.travis.yml

@@ -2,9 +2,7 @@ language: rust
 cache: cargo
 rust: nightly
 before_script:
-  - (test -x $HOME/.cargo/bin/cargo-install-update || cargo install cargo-update)
-  - (test -x $HOME/.cargo/bin/mdbook || cargo install mdbook)
-  - cargo install-update -a
+  - cargo install mdbook -Z install-upgrade
   - mdbook --version
 script:
   - mdbook build