onloadCallback error if no onloadCallback function provided

[wittlesouth]

Getting the following in the console when no onloadCallback function is provided to the control:

Uncaught TypeError: this.props.onloadCallback is not a function

I don't seem to need one for my use case, and therefore this looks like a bug. There are a few spots in the code where there seems to be tests whether this is defined before it is used (e.g. componentDidUpdate), but not in _renderGrecaptcha().