.travis.yml: Add RUSTFLAGS="-D warnings" to env

Have CI fail on warnings.

There's one warning across the workspace in the `salsa20-crate`.
I opened an issue about it here:

#23

I've gone ahead and thrown an `allow` attribute on the relevant code so
CI passes, which should at least prevent additional warnings from going
uncaught on CI.

Author: tarcieri

.travis.yml

@@ -10,6 +10,9 @@ script:
   - cargo test --all --all-features --release
   - ./test_aesni.sh
 
+env:
+  - RUSTFLAGS="-D warnings"
+
 matrix:
   allow_failures:
     - name: rustfmt

salsa20-core/src/salsa_family_state.rs

@@ -93,8 +93,13 @@ pub trait SalsaFamilyCipher {
 
                         self.next_block();
                     } else {
-                        word_idx = 0;
-                        self.next_block();
+                        // TODO(tarcieri): is this else clause unnecessary or is there a bug?
+                        // See: <https://github.com/RustCrypto/stream-ciphers/issues/23>
+                        #[allow(unused_assignments)]
+                        {
+                            word_idx = 0;
+                            self.next_block();
+                        }
                     }
 
                     let nblocks = (datalen - i) / 64;