group_chat_created not in list of supported messages

[marcoceppi]

In order to process an event when a bot joins a new group, group_chat_created needs to be in the list of supported events.

Example of payload:

{
  'update_id': XXXXXX,
  'message': {
    'message_id': 38,
    'from': {
      'id': XXXXXXX,
      'is_bot': False,
      'first_name': 'Marco',
      'last_name': 'Ceppi',
      'username': 'XXXXXXXX',
      'language_code': 'en-US'
    },
    'chat': {
      'id': -XXXXXXX,
      'title': 'Another Test',
      'type': 'group',
      'all_members_are_administrators': True
    },
    'date': 1518477717,
    'group_chat_created': True
  }
}