Handle missing `constraint_name` for sqlite (#73)
Handle missing `constraint_name` for sqlite (#73)

```
SQLite version 3.8.5 2014-08-15 22:37:57
    Enter ".help" for usage hints.
    sqlite> .headers on
    sqlite> .mode column
    sqlite> PRAGMA foreign_key_list ('test_table');
    id          seq         table       from        to          on_update   on_delete   match
    ----------  ----------  ----------  ----------  ----------  ----------  ----------  ----------
```
sdispater/orator,Hanaasagi/sorator