Be explicit for Travis' sake

Author: CAD97

.travis.yml

@@ -1,14 +1,15 @@
 language: rust
-rust:
-  - stable
 
 matrix:
   include:
+
     - rust: nightly-2018-01-26
       before_script:
         - rustup component add rustfmt-preview
       script:
         - cargo fmt -- --write-mode=diff
 
-script:
-  - cargo test
+    - rust:
+        - stable
+      script:
+        - cargo test