beta is not a yarn available version, let's take the latest 2.1.0

package.json

@@ -14,7 +14,7 @@
   },
   "devDependencies": {
     "css-loader": "^0.28.0",
-    "extract-text-webpack-plugin": "beta",
+    "extract-text-webpack-plugin": "2.1.0",
     "file-loader": "^0.11.1",
     "node-sass": "^4.5.2",
     "postcss": "^5.2.17",