Put a 60s limit on the test suite

Nothing should run that long, and when flaky tests hang this is a lot better
than Travis's default 10-minute timeout.

Makefile

@@ -1,7 +1,7 @@
 default: fmt vet errcheck test
 
 test:
-	go test -v -race ./...
+	go test -v -timeout 60s -race ./...
 
 vet:
 	go vet ./...