Update extract-text-webpack-plugin requirement to 3.0.2

Updates the requirements on [extract-text-webpack-plugin](https://github.com/webpack-contrib/extract-text-webpack-plugin) to permit the latest version.
- [Release notes](https://github.com/webpack-contrib/extract-text-webpack-plugin/releases)
- [Changelog](https://github.com/webpack-contrib/extract-text-webpack-plugin/blob/master/CHANGELOG.md)
- [Commits](https://github.com/webpack-contrib/extract-text-webpack-plugin/commits/v3.0.2)

Signed-off-by: dependabot[bot] <support@dependabot.com>

package.json

@@ -36,7 +36,7 @@
     "eslint-plugin-promise": "3.5.0",
     "eslint-plugin-react": "6.10.3",
     "eslint-plugin-standard": "2.1.1",
-    "extract-text-webpack-plugin": "2.1.0",
+    "extract-text-webpack-plugin": "3.0.2",
     "file-loader": "1.1.5",
     "html-webpack-plugin": "2.28.0",
     "jest": "19.0.2",