A stronger check for if the book build in the expected location

Author: berquist

.travis.yml

@@ -11,6 +11,9 @@ before_script:
 
 script:
   - npm run build
+  # Make sure the book built.
+  - |
+      [[ -f "${BOOK_BUILD_DIR}/index.html" ]] || echo "${BOOK_BUILD_DIR}/index.html not found" && exit 1
 
 after_success:
     - |