docs(readme): update url for package logo

Author: joshwiens

README.md

@@ -7,7 +7,7 @@
 
 <div align="center">
   <img width="200" height="200"
-    src="https://webpack.github.io/extract-text-webpack-plugin/logo.svg">
+    src="https://cdn.rawgit.com/webpack-contrib/extract-text-webpack-plugin/574e3200/logo.svg">
   <a href="https://github.com/webpack/webpack">
     <img width="200" height="200"
       src="https://webpack.js.org/assets/icon-square-big.svg">