Update moment-node.d.ts

According to http://momentjs.com/docs/#/parsing/is-valid/, there is a invalidAt() method, which is missing in this definition file.

Source: https://github.com/moment/moment/blob/develop/src/lib/moment/valid.js#L13

Thanks,

moment/moment-node.d.ts

@@ -401,6 +401,7 @@ declare module moment {
         unix(timestamp: number): Moment;
 
         invalid(parsingFlags?: Object): Moment;
+        invalidAt(): number;
         isMoment(): boolean;
         isMoment(m: any): boolean;
         isDate(m: any): boolean;