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

@@ -22,7 +22,7 @@
     "babel-preset-es2015": "^6.24.0",
     "babel-preset-react": "^6.23.0",
     "css-loader": "^0.28.11",
-    "extract-text-webpack-plugin": "^2.1.0",
+    "extract-text-webpack-plugin": "^3.0.2",
     "file-loader": "^1.1.11",
     "flumpt": "^0.2.0",
     "material-ui": "^0.20.1",