Bench only in nightly.

Author: emilio

.travis.yml

@@ -10,8 +10,8 @@ script:
   - cargo doc --verbose
   - cargo test --features heapsize
   - cargo test --features dummy_match_byte
-  - cargo bench --features bench
-  - cargo bench --features "bench dummy_match_byte"
+  - if [ "$TRAVIS_RUST_VERSION" == "nightly" ]; then cargo bench --features bench; fi
+  - if [ "$TRAVIS_RUST_VERSION" == "nightly" ]; then cargo bench --features "bench dummy_match_byte"; fi
 
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