Update extract-text-webpack-plugin to latest (#1700)

Author: SimenB

packages/react-scripts/package.json

@@ -44,7 +44,7 @@
     "eslint-plugin-import": "2.0.1",
     "eslint-plugin-jsx-a11y": "2.2.3",
     "eslint-plugin-react": "6.4.1",
-    "extract-text-webpack-plugin": "2.0.0-rc.3",
+    "extract-text-webpack-plugin": "2.0.0",
     "file-loader": "0.10.0",
     "filesize": "3.3.0",
     "fs-extra": "0.30.0",