Check formatting in CI

.travis.yml

@@ -1,15 +1,25 @@
 language: rust
+sudo: false
 rust:
   - 1.8.0
   - 1.15.0
   - 1.20.0
+  - 1.26.0 # has_i128
+  - 1.31.0 # 2018!
   - stable
   - beta
   - nightly
-sudo: false
 script:
   - cargo build --verbose
   - ./ci/test_full.sh
+matrix:
+  include:
+    - name: "rustfmt"
+      rust: 1.31.0
+      before_script:
+        - rustup component add rustfmt
+      script:
+        - cargo fmt --all -- --check
 notifications:
   email:
     on_success: never