Fix clicking modal background to close in IOS

Fixes: https://github.com/react-bootstrap/react-bootstrap/issues/150

I wonder if this should be worked around in React itself?
We should probably be careful not to touch the DOM on every render in the tree, something like this should be enough:
```
componentDidUpdate: function (prevProps) {
  if (this.props.backdrop === true && this.props.backdrop !== prevProps.backdrop) {
    this.iosClickHack();
  }
}
```