.travis.yml: use go vet instead of go tool vet

As of Go 1.12, go tool vet is no longer supported¹.
Start using the go vet command instead.

¹ https://golang.org/doc/go1.12#vet

.travis.yml

@@ -12,5 +12,5 @@ install:
 script:
   - go get -t -v ./...
   - diff -u <(echo -n) <(gofmt -d -s .)
-  - go tool vet .
+  - go vet ./...
   - go test -v -race ./...