In TwimlCallback, Node shouldn't be optional

Because the callback is always called with the Node.

This change is based on the advice here:
https://github.com/Microsoft/TypeScript-Handbook/blob/master/pages/decla
ration%20files/Do's%20and%20Don'ts.md#optional-parameters-in-callbacks

twilio/twilio.d.ts

@@ -381,7 +381,7 @@ declare namespace twilio {
 
   export interface TwimlMethod { (arg1: any | string | TwimlCallback, arg2?: any | string | TwimlCallback): Node }
 
-  export interface TwimlCallback { (node?: Node): void; }
+  export interface TwimlCallback { (node: Node): void; }
 
   export class Node implements NodeOptions {
     name: string;