refactor: use static values for cache scope

This fixes a future problem when code is placed under a namespace because `get_class($bridge)` will then return e.g. `RssBridge\Bridge\TwitterBridge` instead of the the current value `TwitterBridge`.

Also a bit refactoring of `Configuration.php`.

bridges/ElloBridge.php

@@ -116,7 +116,7 @@ class ElloBridge extends BridgeAbstract
         $cacheFactory = new CacheFactory();
 
         $cache = $cacheFactory->create();
-        $cache->setScope(get_called_class());
+        $cache->setScope('ElloBridge');
         $cache->setKey(['key']);
         $key = $cache->loadData();