ci: ugh. bash. why.

.travis.yml

@@ -13,7 +13,7 @@ script:
   # Fast-fail on non-zero exit codes
   - set -e
   # Verify that all files have been gofmt'd with simplification.
-  - diff <(go fmt ./... 2>&1) <(printf "")
+  - diff <(GOFLAGS="-mod-vendor" go fmt ./... 2>&1) <(printf "")
   # Verify that unit tests pass
   - go test -v ./...