.travis.yml: Allow warnings on 1.27.0

There are several `generic-array`-related warnings on 1.27.0:

https://travis-ci.org/RustCrypto/stream-ciphers/jobs/571083202

We should probably phase 1.27.0 out anyway (#24), so ignore them.

Author: tarcieri

.travis.yml

@@ -22,6 +22,7 @@ matrix:
   include:
     - name: "Rust: 1.27.0"
       rust: 1.27.0
+      env: {} # clear `-D warnings` above; allow warnings
       script:
         - cargo test --all --release
         - ./test_aesni.sh