Fix invalid option of cargo fmt

Author: hyunsik

.travis.yml

@@ -2,6 +2,6 @@ language: rust
 before_script:
   - rustup component add rustfmt-preview
 script:
-  - cargo fmt --all -- --write-mode=diff
+  - cargo fmt --all -- --check
   - cargo build --verbose --all
   - cargo test --verbose --all