Use canonical imports (#822)

* fix: challenge are not required for revoke.

cmd/cmd_renew.go

@@ -1,4 +1,4 @@
-package cmd
+package cmd // import "github.com/xenolf/lego/cmd"
 
 import (
 	"crypto"
@@ -53,6 +53,7 @@ func createRenew() cli.Command {
 
 func renew(ctx *cli.Context) error {
 	account, client := setup(ctx, NewAccountsStorage(ctx))
+	setupChallenges(ctx, client)
 
 	if account.Registration == nil {
 		log.Fatalf("Account %s is not registered. Use 'run' to register a new account.\n", account.Email)