enforce rustfmt on travis

.travis.yml

@@ -17,10 +17,12 @@ install:
   - nvm install stable
   - nvm use stable
   - npm install remark-cli remark-lint
+  - cargo install rustfmt
 
 script:
  - remark -f README.md > /dev/null
  - python util/update_lints.py -c
+ - cargo fmt -- --write-mode=diff
  - set -e
  - cargo build --features debugging
  - cargo test --features debugging