fix slow get_is_sortable when using with inline admins

the first `if` call did execute the whole `select` query with (perhaps)
millions of records in them, even though the only result needed here
is one record.

The acually used queryset later will be filtered by the parent-model.

adminsortable/utils.py

@@ -6,12 +6,14 @@ def check_inheritance(obj):
 
 
 def get_is_sortable(objects):
-    if objects:
-        if check_inheritance(objects[0]):
-            if objects.count() > 1:
-                return True
+    if objects.count() < 2:
         return False
 
+    if not check_inheritance(objects[:1][0]):
+        return False
+
+    return True
+
 
 def is_self_referential(cls):
     cls_type = type(cls)