Test both nightly and stable in Travis

I can't tell why a nightly build didn't exist before. Travis was
silently only building on stable.

Author: dtolnay

.travis.yml

@@ -1,12 +1,8 @@
 language: rust
 
-rust:
-  - nightly
-
-script:
-  - cargo test
-
 matrix:
   include:
+    - rust: nightly
+      script: cargo test
     - rust: stable
       script: cargo check