react-this-binding-issue should ignore ref callbacks

In a react render function, react-this-binding-issue fires on `ref` when it should not.  E.g.:

```
class Form extends React.Component<{action: string}, {}> {
  render() {
    return (
      <form
         ref={f => (this.form = f)} // lint error here
         action={this.props.action}
         >
         {this.props.children}
      </form>
    );
  }
}
```