fix ref (fixes #2)

[ngokevin]

No description provided.

[cipherchien]

I still get the same error message.
I think the problem should be line 120:

`return _react2['default'].createElement(
......

ref: function (div) {
_this2.input = div.querySelector('input');
}
},
_react2['default'].createElement('input', _extends({}, this.props, { children: undefined, type: 'file',
...
`

When I mark "ref". the error message is disappear.

the error message told me:
Uncaught Error: Invariant Violation: addComponentAsRefTo(...): Only a ReactOwner can have refs. You might be adding a ref ....

So I think that is the point of problem.

[maximerety]

I'm closing this old PR since it has been fixed in another one (#6).