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

As of Go 1.12, go tool vet is no longer supported.¹
Start using the go vet command instead.

Use RCS format for gofmt diff. It results in cleaner,
easier to read gofmt diffs.

¹ https://golang.org/doc/go1.12#vet

.travis.yml

@@ -15,6 +15,6 @@ install:
   - # Do nothing. This is needed to prevent default install action "go get -t -v ./..." from happening here (we want it to happen inside script step).
 script:
   - go get -t -v ./...
-  - diff -u <(echo -n) <(gofmt -d -s .)
-  - go tool vet .
+  - diff -n <(echo -n) <(gofmt -d -s .)
+  - go vet ./...
   - go test -v -race ./...