Add Rails plugin dirs to Webpack modulesDirectories

This should allow Webpack to bundle assets from Rails plugins/gems,
as if they were npm modules or Bower components, but without us
having to add an entry to core's `package.json`/`bower.json`.

Signed-off-by: Alex Coles <alex@alexbcoles.com>

webpack.config.js

@@ -26,7 +26,7 @@ module.exports = {
     modulesDirectories: [
       'node_modules',
       'vendor/assets/components'
-    ],
+    ].concat(pathConfig.pluginDirectories),
 
     alias: {
       'locales':        'config/locales',