build: 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

@@ -76,7 +76,7 @@
     "cpy-cli": "1.0.1",
     "css-loader": "0.23.1",
     "exports-loader": "0.6.3",
-    "extract-text-webpack-plugin": "2.0.0-beta.1",
+    "extract-text-webpack-plugin": "3.0.2",
     "file-loader": "0.9.0",
     "ghooks": "2.0.3",
     "html-loader": "0.4.3",