Deny warnings on Travis
diff --git a/.travis.yml b/.travis.yml
index b63b4a0..3e67b0f 100644
--- a/.travis.yml
+++ b/.travis.yml
@@ -4,6 +4,10 @@
 
 rust: nightly
 
+env:
+  - RUSTFLAGS="-D warnings"
+  - CARGO_TARGET_X86_64_UNKNOWN_LINUX_GNU_RUNNER="valgrind --leak-check=full --error-exitcode=1"
+
 addons:
   apt:
     packages:
@@ -15,5 +19,4 @@
 script:
   - cargo fmt --all -- --check
   - cargo check --benches --bins --examples --tests
-  - CARGO_TARGET_X86_64_UNKNOWN_LINUX_GNU_RUNNER="valgrind --leak-check=full --error-exitcode=1"
-    cargo test -- --test-threads=1
+  - cargo test -- --test-threads=1