tech(build): require go vet to pass

scripts/circle-test.sh

@@ -20,7 +20,7 @@ exit_if_fail npm test
 
 exit_if_fail test -z "$(gofmt -s -l ./pkg | tee /dev/stderr)"
 exit_if_fail go run build.go build
-exit_if_fail go vet ./pkg/...
+exit_if_fail test -z "$(go vet ./pkg/... | tee /dev/stderr)"
 exit_if_fail go test -v ./pkg/...