Update to the new rustfmt cli

Author: lu-zero

.travis.yml

@@ -48,7 +48,7 @@ matrix:
       before_script:
       - rustup component add rustfmt-preview
       script:
-      - cargo fmt --all -- --write-mode=diff
+      - cargo fmt --all -- --check
     - env: CLIPPY=On TARGET=x86_64-unknown-linux-gnu NO_ADD=1
       script: |
         cargo install clippy