Fix mini-css-extract-plugin 0.4.3 issue

Resolves shellscape#167

mini-css-extract-plugin reports additional, incorrect information for files that are refenced in CSS. The first time we see the file the `module.userRequest` is correct, and we add to `moduleAssets` correctly. However mini-css-extract-plugin then also reports the same `file` but with `module.userRequest` set to the CSS file that references it, which caused us to overwrite the good value in `moduleAssets`.

See the change in mini-css-extract-plugin that caused this webpack-contrib/mini-css-extract-plugin#177

lib/plugin.js

@@ -40,7 +40,7 @@ ManifestPlugin.prototype.apply = function(compiler) {
   var outputName = path.relative(outputFolder, outputFile);
 
   var moduleAsset = function (module, file) {
-    if (module.userRequest) {
+    if (module.userRequest && !moduleAssets[file]) {
       moduleAssets[file] = path.join(
         path.dirname(file),
         path.basename(module.userRequest)