Use the latest beta of the extract-text-webpack-plugin dependency (compatibility with webpack 4)

Author: DominikPalo

package.json

@@ -32,7 +32,7 @@
   "devDependencies": {
     "cheerio": "^0.22.0",
     "css-loader": "^0.25.0",
-    "extract-text-webpack-plugin": "^3.0.2",
+    "extract-text-webpack-plugin": "^4.0.0-beta.0",
     "html-webpack-plugin": "^4.0.0-beta.4",
     "jasmine": "^2.4.1",
     "rimraf": "^2.5.2",