Check with deadlinks

.travis.yml

@@ -11,12 +11,18 @@ addons:
     - gcc-6
     - g++-6
     - cmake
+install:
+- cargo install cargo-deadlinks
 script:
 - export CC=/usr/bin/gcc-6
 - export CXX=/usr/bin/g++-6
 - cargo test --verbose
 - cargo test --manifest-path=spec/Cargo.toml
+# cargo-deadlinks will check any links in docs generated by `cargo doc`.
+# This is useful as rustdoc uses raw links which is error prone.
+- cargo doc && cargo deadlinks
 after_success: |
+  # Build documentation and deploy it to github pages.
   [ $TRAVIS_BRANCH = master ] &&
   [ $TRAVIS_PULL_REQUEST = false ] &&
   cargo doc &&