npm: Upgrade webpack related development dependencies

compression-webpack-plugin fixes https://www.npmjs.com/advisories/1548

Adjust to new copy-webpack-plugin API.

Thew new modules now create dist/index.html.gz in `NODE_ENV=production`
mode, so switch `WEBPACK_TEST` to index.css instead (which remains
uncompressed in both modes).

webpack.config.js

@@ -78,7 +78,7 @@ info.files.forEach(function(value) {
 info.files = files;
 
 var plugins = [
-    new copy(info.files),
+    new copy({ patterns: info.files }),
     new extract({filename: "[name].css"})
 ];