.verifyCallback() does not work with es6 classes

Hello,

When calling the `.execute()` method on the recaptcha instance, the `verifyCallback` function is not called if within an es6 class. The `verifyCallback` is only called when outside the class. This is not ideal as it doesn't allow you to submit a form.

**Doesn't work**

import * as React from 'react';
import { RouteComponentProps } from 'react-router';
var Recaptcha = require('react-recaptcha');

export class Submit extends React.Component, {}>{
recaptchaInstance: any;
constructor() {
super();

this.executeCaptcha = this.executeCaptcha.bind(this);
this.verifyCallback = this.verifyCallback.bind(this);
}

executeCaptcha() {
this.recaptchaInstance.execute();
}

verifyCallback(response){
console.log(response); // never is called
// submit form
}

render() {
return

It _does_ work this way

import * as React from 'react';
import { RouteComponentProps } from 'react-router';
var Recaptcha = require('react-recaptcha');

const verifyCallback = function(response){
console.log(response);
// can't submit form now!? (or is there a way)
}

export class Submit extends React.Component, {}>{
recaptchaInstance: any;
constructor() {
super();

this.executeCaptcha = this.executeCaptcha.bind(this);
this.verifyCallback = this.verifyCallback.bind(this);
}

executeCaptcha() {
this.recaptchaInstance.execute();
}

verifyCallback(response){
console.log(response); // never is called
}

render() {
return

Is there a way we can stick the `verifyCallback` in the class?