require-extension: false warnings when npm package name ends with .js

`require('headroom.js')` requires [npm package headroom.js](https://www.npmjs.com/package/headroom.js) and should be considered valid by the [require-extension](https://github.com/yannickcr/eslint-plugin-react/blob/master/docs/rules/require-extension.md) rule. Right now it triggers a warning.
