Looks like the latest mdbook release isn't compatible with japaric/trust, build from source

Author: Michael-F-Bryan

.travis.yml

@@ -13,8 +13,8 @@ before_script:
     - |
       (travis_wait rustup component add rustfmt-preview || true) &&
       (travis_wait rustup component add clippy-preview || true)
-      # install mdbook on nightly builds for the user guide
-    - curl -LSfs https://japaric.github.io/trust/install.sh | sh -s -- --git rust-lang-nursery/mdBook --force
+      # install mdbook for the user guide
+    - cargo install --debug mdbook
 
 script:
     - |