Sort children when deserializing DAG

[jedcunningham]

MySQL doesn't maintain the order of children round trip, so this keeps the deserialized DAGs consistent across db types.

e.g. MySQL:

MySQL [airflow]> create table foo (data json);
Query OK, 0 rows affected (0.026 sec)

MySQL [airflow]> insert into foo values ('{"hello": "world", "ab": "cb"}');
Query OK, 1 row affected (0.003 sec)

MySQL [airflow]> select * from foo;
+--------------------------------+
| data                           |
+--------------------------------+
| {"ab": "cb", "hello": "world"} |
+--------------------------------+
1 row in set (0.000 sec)

and PostgreSQL:

postgres@localhost:postgres> create table foo (data json);
CREATE TABLE
Time: 0.005s
postgres@localhost:postgres> insert into foo values ('{"hello": "world", "ab": "cb"}');
INSERT 0 1
Time: 0.003s
postgres@localhost:postgres> select * from foo;
+--------------------------------+
| data                           |
|--------------------------------|
| {"hello": "world", "ab": "cb"} |
+--------------------------------+
SELECT 1
Time: 0.010s
postgres@localhost:postgres>