tlsconst: Update TLS cipher suites, and include TLS 1.3

This patch updates the list of known TLS cipher suites, and adds TLS 1.3
to the list of known versions (it will be included in Go 1.12).

internal/tlsconst/tlsconst.go

@@ -11,6 +11,7 @@ var versionName = map[uint16]string{
 	0x0301: "TLS-1.0",
 	0x0302: "TLS-1.1",
 	0x0303: "TLS-1.2",
+	0x0304: "TLS-1.3",
 }
 
 // VersionName returns a human-readable TLS version name.