fixing missing flush event handler

In [commit](https://github.com/FriendsOfSymfony/FOSElasticaBundle/commit/843c76b6cabd0fb71ef03cd95b9702e9dd41b2fc#diff-850942b3ba24ab03a40aaa81b6152852) the configuration-definition for the flush listener was accidentally removed. 
As the flush listener is no longer set to be enabled in the extensions getDoctrineEvents method, the flush listener is not set. 
This results in a situation were we are only able to have the modified objects on the list for index-update, but never actually sending the update to the ES host.