Update fields.py

When setting JSONFIELD_CUSTOM the logic was importing under the wrong name with `Base` suffix raising `Import Error` as the module ended up having no `JSONField` to be imported.

Looks like due toa  recent change the JSONFieldBase` is no more needed.

social_django/fields.py

@@ -21,7 +21,7 @@
             from django.utils.module_loading import import_string
         except ImportError:
             from importlib import import_module as import_string
-        JSONFieldBase = import_string(JSONFIELD_CUSTOM)
+        JSONField = import_string(JSONFIELD_CUSTOM)
     else:
         try:
             from django.db.models import JSONField