Add a (nonfailing) rustfmt check to the CI

Author: josephlr

.travis.yml

@@ -137,6 +137,14 @@ matrix:
       script:
         - bash utils/ci/script.sh
 
+  allow_failures:
+    - rust: stable
+      env: DESCRIPTION="Rustfmt"
+      install:
+        - rustup component add rustfmt
+      script:
+        - cargo fmt --all -- src/*.rs --check
+
 before_install:
   - set -e
   - rustup self update