fix(package.json): update Webpack 2.0 breaking extract-text-webpack dependency

Author: niieani

package.json

@@ -41,7 +41,7 @@
   },
   "dependencies": {
     "css-loader": "^0.23.1",
-    "extract-text-webpack-plugin": "^1.0.1",
+    "extract-text-webpack-plugin": "^2.0.0-beta",
     "resolve-url-loader": "^1.4.4",
     "sass-loader": "^4.0.0",
     "style-loader": "^0.13.1",