Added cargo-fmt to CI runs (#627)

* Added cargo-fmt to CI runs

Closes #625

* Remove fmt from appveyor builds

Windows seems to have issues with install components through rustup.

Formatting should be equally informative regardless of the OS,
so best to keep it in Linux on Travis

.travis.yml

@@ -47,6 +47,7 @@ matrix:
 before_install:
   - set -e
   - rustup self update
+  - rustup component add rustfmt
 
 install:
   - sh ci/install.sh
@@ -60,6 +61,7 @@ before_script:
 
 script:
   - bash ci/script.sh
+  - cargo fmt --all -- --check
 
 before_deploy:
   - sh ci/before_deploy.sh