Travis: require build on stable Rust to succeed.

.travis.yml

@@ -13,8 +13,6 @@ matrix:
       env: FEATURES='alloc collections' MODE='build'
     - rust: nightly
       env: FEATURES='' MODE='build'
-  allow_failures:
-    - rust: stable # until 1.15 comes out
 script:
    - cargo "$MODE" --no-default-features --features "$FEATURES"
 notifications: