You are getting this error because Laravel 5 uses namespacing by default. The official Laravel 5 upgrade guide says the following about migrating your controllers:
Since we are not going to migrate to full namespacing in this guide, add the app/Http/Controllers directory to the classmap directive of your composer.json file. Next, you can remove the namespace from the abstract app/Http/Controllers/Controller.php base class. Verify that your migrated controllers are extending this base class.
In your app/Providers/RouteServiceProvider.php file, set the namespace property to null.
Listed here under "controllers".
The last line is probably the one that will solve your issue.