Run rustfmt in travis.

Author: tomprince

.travis.yml

@@ -1,5 +1,6 @@
 sudo: required
 language: rust
+cache: cargo
 addons:
   apt:
     packages:
@@ -16,10 +17,12 @@ before_script:
   - |
     pip install 'travis-cargo<0.2' --user &&
     export PATH=$HOME/.local/bin:$PATH
+  - cargo install --force rustfmt
 script:
   - |
     travis-cargo build &&
     travis-cargo test &&
+    travis-cargo fmt -- --write-mode diff &&
     travis-cargo --only stable doc
 after_success:
   - travis-cargo coveralls --no-sudo --verify