Fix the last warning

Author: sfackler

.circleci/config.yml

@@ -28,6 +28,8 @@ jobs:
     working_directory: ~/build
     docker:
       - image: rust:1.19.0
+        environment:
+          RUSTFLAGS: -D warnings
       - image: sfackler/rust-postgres-test:3
     steps:
       - checkout