Update mixed env message for Webpack 4

Author: timdorr

src/index.js

@@ -18,10 +18,10 @@ if (
   isCrushed.name !== 'isCrushed'
 ) {
   warning(
-    "You are currently using minified code outside of NODE_ENV === 'production'. " +
+    'You are currently using minified code outside of NODE_ENV === "production". ' +
       'This means that you are running a slower development build of Redux. ' +
       'You can use loose-envify (https://github.com/zertosh/loose-envify) for browserify ' +
-      'or DefinePlugin for webpack (http://stackoverflow.com/questions/30030031) ' +
+      'or setting mode to production in webpack (https://webpack.js.org/concepts/mode/) ' +
       'to ensure you have the correct code for your production build.'
   )
 }