Change filter_taggings table creation

I'm not entirely sure what sorcery this is, just that I was getting this fun error when I tried to create a work on my local install:

```
Mysql2::Error: Field 'id' doesn't have a default value: INSERT INTO `filter_taggings` 
(`created_at`, `filter_id`, `filterable_id`, `filterable_type`, `inherited`, `updated_at`) 
VALUES ('2014-03-22 02:35:33', 992891255, 108, 'Work', 0, '2014-03-22 02:35:33')
```

And the table looked like this:
```
Field           | Type         | Null | Key | Default | Extra |
+-----------------+--------------+------+-----+---------+-------+
| id              | int(11)      | NO   |     | NULL    |       |
```

Elz had me change this line in the schema to make the error go away and turn the table into this:
```
 Field           | Type         | Null | Key | Default | Extra          |
+-----------------+--------------+------+-----+---------+----------------+
| id              | int(11)      | NO   | PRI | NULL    | auto_increment |
|
```

When I asked what to do with the altered file, she said check it out or pull request it, so here we are.