run cargo audit with --deny warnings enabled

It is better to fail on warnings as well when running `cargo
audit` so we can catch problems in time.
Fixes #52.

Signed-off-by: Laura Loghin <[email protected]>

.buildkite/test_description.json

@@ -71,7 +71,7 @@
     },
     {
       "test_name": "cargo-audit",
-      "command": "cargo audit -q"
+      "command": "cargo audit -q --deny warnings"
     }
   ]
 }