Make Travis test against multiple Rust versions, including Rust 1.25.

.travis.yml

@@ -1,3 +1,13 @@
 language: rust
+rust:
+  - 1.25.0  # Must align with `build/version.rs`.
+  - stable
+  - beta
+  - nightly
+matrix:
+  allow_failures:
+    - rust: nightly
+  fast_finish: true
 script:
   - cargo test --verbose --all
+cache: cargo