let nightly Rust always pass Travis' checks

.travis.yml

@@ -16,6 +16,11 @@ before_script:
     - rustup component add rustfmt
 
 script:
-    - cargo build --locked --verbose
-    - cargo test --locked --verbose
-    - cargo fmt --all -- --check
+   - cargo build --all --locked --verbose
+   - cargo test --all --locked --verbose
+   - cargo fmt --all -- --check
+
+jobs:
+  allow_failures:
+    - rust: nightly
+  fast_finish: true