all: update to go1.13 release

Change-Id: Id090455ddf30620bd5be809a095d187afc2047c5
Reviewed-on: https://go-review.googlesource.com/c/protobuf/+/193937
Reviewed-by: Joe Tsai <thebrokentoaster@gmail.com>

integration_test.go

@@ -34,7 +34,7 @@ var (
 	buildRelease = flag.Bool("buildRelease", false, "build release binaries")
 
 	protobufVersion = "3.9.1"
-	golangVersions  = []string{"1.9.7", "1.10.8", "1.11.13", "1.12.9", "1.13rc1"}
+	golangVersions  = []string{"1.9.7", "1.10.8", "1.11.13", "1.12.9", "1.13"}
 	golangLatest    = golangVersions[len(golangVersions)-1]
 
 	// purgeTimeout determines the maximum age of unused sub-directories.