Use Transport.ForceAttemptHTTP2 to enable HTTP/2

This can remove `golang.org/x/net/http2` dependency.
Ref: https://golang.org/doc/go1.13#net/http

README.md

@@ -9,7 +9,7 @@ Imitation is the sincerest form of flattery.
 But seriously, https://github.com/reorx/httpstat is the new hotness, and this is a shameless rip off.
 
 ## Installation
-`httpstat` requires Go 1.11 or later.
+`httpstat` requires Go 1.13 or later.
 ```
 $ go get github.com/davecheney/httpstat
 ```	

main.go

@@ -22,8 +22,6 @@ import (
 	"strings"
 	"time"
 
-	"golang.org/x/net/http2"
-
 	"github.com/fatih/color"
 )
 
@@ -253,6 +251,7 @@ func visit(url *url.URL) {
 		IdleConnTimeout:       90 * time.Second,
 		TLSHandshakeTimeout:   10 * time.Second,
 		ExpectContinueTimeout: 1 * time.Second,
+		ForceAttemptHTTP2:     true,
 	}
 
 	switch {
@@ -274,13 +273,6 @@ func visit(url *url.URL) {
 			InsecureSkipVerify: insecure,
 			Certificates:       readClientCert(clientCertFile),
 		}
-
-		// Because we create a custom TLSClientConfig, we have to opt-in to HTTP/2.
-		// See https://github.com/golang/go/issues/14275
-		err = http2.ConfigureTransport(tr)
-		if err != nil {
-			log.Fatalf("failed to prepare transport for HTTP/2: %v", err)
-		}
 	}
 
 	client := &http.Client{