[BridgeAbstract] replace getCacheDuration method by CACHE_TIMEOUT class constant

Signed-off-by: Pierre Mazière <pierre.maziere@gmx.com>

bridges/TwitterBridge.php

@@ -2,6 +2,7 @@
 class TwitterBridge extends BridgeAbstract{
     const NAME='Twitter Bridge';
     const URI='https://twitter.com/';
+    const CACHE_TIMEOUT = 300; // 5min
     const DESCRIPTION='returns tweets';
     const PARAMETERS=array(
         'global'=>array(
@@ -135,8 +136,4 @@ EOD;
 			$this->items[] = $item;
 		}
 	}
-
-	public function getCacheDuration(){
-		return 300; // 5 minutes
-	}
 }