AttributeError: 'bool' object has no attribute 'is_pk' in migrator
The problem, not obvious at all, is that the `ticket` column is not found. I bet that if you change it to `ticket_id` you will be set:

``` python
from playhouse.migrate import MySQLMigrator, migrate
migrator = MySQLMigrator(db.database)
migrate(migrator.drop_not_null('emailsending', 'ticket_id'))
```
 Looking at the tests, there may be some additional issues with the mysql migrations regarding dropping constraints on foreign keys. Looking into it.
 @coleifer - Seeing that you closed this issue I presume that after you looked into it you concluded that there were no additional issues with the mysql migrations regarding dropping constraints on foreign keys?

The reason I'm asking is that I'm using this in beta/production now and I wanted to wait with migrating the db until you had a conclusion.
 Just to be safe I added another testcase adding and dropping not null constraints to a foreign key to verify they worked.
 Thanks for adding the tests. I tested it on 3 test setups before deploying the migration to the production server, and everything went perfectly fine.

Thanks for creating peewee!

I've successfully used the peewee migrator to add and drop columns, and today I wanted to use the migrator for the first time to drop a not null constrain on a column. In my model I added and set the null constraint to `True`:

```
ticket = ForeignKeyField(Ticket, related_name='sent_to_email_addresses', null=True)
```

So I wrote the following lines to do the migration:

```
from playhouse.migrate import MySQLMigrator, migrate
migrator = MySQLMigrator(db.database)
migrate(migrator.drop_not_null('emailsending', 'ticket'))
```

But I'm getting the following error:

```
File "./migrate_db.py", line 55, in from7To8
    migrate(migrator.drop_not_null('emailsending', 'ticket'))
  File "/usr/local/lib/python2.7/dist-packages/playhouse/migrate.py", line 579, in migrate
    operation.run()
  File "/usr/local/lib/python2.7/dist-packages/playhouse/migrate.py", line 144, in run
    getattr(self.migrator, self.method)(*self.args, **kwargs))
  File "/usr/local/lib/python2.7/dist-packages/playhouse/migrate.py", line 152, in inner
    return fn(self, *args, **kwargs)
  File "/usr/local/lib/python2.7/dist-packages/playhouse/migrate.py", line 390, in drop_not_null
    if column.is_pk:
AttributeError: 'bool' object has no attribute 'is_pk'
```

Does anybody know what I'm doing wrong here?
