only enable compatible apps

lib/app.php

@@ -189,8 +189,16 @@ class OC_App{
 			}
 		}
 		if($app!==false){
-			OC_Appconfig::setValue( $app, 'enabled', 'yes' );
-			return true;
+             		// check if the app is compatible with this version of ownCloud
+			$info=OC_App::getAppInfo($app);
+			$version=OC_Util::getVersion();	
+	                if(!isset($info['require']) or ($version[0]>$info['require'])){
+				OC_Log::write('core','App can\'t be installed because it is not compatible with this version of ownCloud',OC_Log::ERROR);
+				return false;
+			}else{
+				OC_Appconfig::setValue( $app, 'enabled', 'yes' );
+				return true;
+			}
 		}else{
 			return false;
 		}