(hotfix): base_manager_name should be 'objects'

- the Django docs for 1.10+ on this don't give an example, so I ended
  up incorrectly adding the name of the manager class, but they're
  actually looking for the name of the attribute in the class that
  contains the instance of the manager 😅
  - see https://stackoverflow.com/a/48124863/3431180 for an example
  - see #4
    for discussion / bug report

Author: agilgur5

django_serializable_model.py

@@ -47,7 +47,7 @@ class SerializableModel(models.Model):
     class Meta:
         abstract = True
         # when queried from a related Model, use this Manager
-        base_manager_name = 'SerializableManager'
+        base_manager_name = 'objects'
 
     def serialize(self, *args, **kwargs):
         """