inputRef should support React.createRef #176

[mihaimartalogu]

For cases when custom components reference ReactS3Uploader's input through a React.createRef(), for example:

  const uploaderInputRef = React.createRef();
  ...
      <ReactS3Uploader
        ...
        inputRef={uploaderInputRef}
      />
      <button onClick={() => { uploaderInputRef.current.click(); }}>upload file</button>

[rootux]

Looks good. @odysseyscience please merge.