travis: Add cargo fmt (#184)

Instructions are copied from the rustfmt README: https://github.com/rust-lang/rustfmt

Author: kevinji

.travis.yml

@@ -10,6 +10,9 @@ notifications:
   email:
     - [email protected]
     - [email protected]
+before_script:
+    - rustup component add rustfmt
 script:
     - cargo build --verbose
     - cargo test --verbose
+    - cargo fmt -- --check