"require-optimization" shouldn't warn on stateless components

Configuration:

``` json
"rules": {
  "react/require-optimization": 2
}
```

Code:

``` js
import React from 'react';
import styles from './Spinner.css';

function Spinner() { // Component is not optimized. Please add a shouldComponentUpdate method
  return <div className={styles.spinner} />;
}

export default Spinner;
```
