Check that code conforms to rustfmt formatting

Author: baumanj

.travis.yml

@@ -13,6 +13,9 @@ env:
     - RELEASE=
     - RELEASE=--release
 
+before_script:
+- rustup component add rustfmt
+
 script:
   - cargo test --all --verbose $RELEASE
   # We cannot run `cargo test` with the mp4parse_infallible feature enabled
@@ -23,3 +26,4 @@ script:
   # See https://github.com/mozilla/mp4parse-rust/issues/197
   - cargo check --all --verbose $RELEASE --tests --all-features
   - cargo doc --package mp4parse_capi
+  - cargo fmt -- --check || echo "Please reformat your code with 'cargo fmt' (version $(cargo fmt -- --version))"