Serializer doesn't work in django 2

[M1ha-Shvn]

Hi.
rel attribute is deprecated in django 2.0. So serializer is broken.
Most probably rel should be replaced with remote_field attribute for Django 2.0 support.

  File "/home/vagrant/venv/lib/python3.7/site-packages/django_elasticsearch/managers.py", line 46, in wrapper
    return f(*args, **kwargs)
  File "/home/vagrant/venv/lib/python3.7/site-packages/django_elasticsearch/managers.py", line 104, in serialize
    return serializer.serialize(self.instance)
  File "/home/vagrant/venv/lib/python3.7/site-packages/django_elasticsearch/serializers.py", line 167, in serialize
    return json.dumps(self.format(instance),
  File "/home/vagrant/venv/lib/python3.7/site-packages/django_elasticsearch/serializers.py", line 154, in format
    for field in fields])
  File "/home/vagrant/venv/lib/python3.7/site-packages/django_elasticsearch/serializers.py", line 154, in <listcomp>
    for field in fields])
  File "/home/vagrant/venv/lib/python3.7/site-packages/django_elasticsearch/serializers.py", line 118, in serialize_field
    if field.rel:
AttributeError: 'IntegerField' object has no attribute 'rel'